The phrase to add insult to injury means: to act in a way that makes matters worse in a bad situation or when somebody has already been hurt or upset.

This phrase occurs, for example, in UK minister departs Cop28 as climate talks reach crisis point, by Fiona Harvey, Patrick Greenfield and Helena Horton, published in The Guardian (London and Manchester, England) of 12^th December 2023:

The Green MP Caroline Lucas said: “The government’s last shred of moral authority in tackling the climate emergency has been obliterated by this scandalous decision to leave Cop28 negotiations at the most critical moment.
“Adding insult to injury, if true that the minister is leaving the summit in order to vote in favour of the utterly immoral Rwanda deal, it shows that Rishi Sunak prioritises saving his own skin over saving the planet.”

The earliest occurrences of the phrase to add insult to injury, also to add insult to injuries, that I have found are as follows, in chronological order—I have included a few occurrences of variants such as to add insult to injustice and to add insult to oppression:

1-: From a memorial written by the King of Prussia, relating to a dispute with the Prince of Liege concerning the lordship of Herstal—published in The Scots Magazine (Edinburgh, Midlothian, Scotland) of September 1740:

His Majesty sent a Privy-counsellor (Rambonet) with a commission to demand a categorical answer of the Prince of Liege, whether he intended to persist in his claim to the sovereignty over the lordship of Herstal, and to support the rebels, or whether he would absolutely decline it, and abandon the authors of the rebellion? To which the said Prince has not vouchsafed any answer.
This was adding insult to injustice, and directly attacking the King’s honour. His Majesty has therefore been forced by the Prince of Liege to part with his usual moderation upon this occasion, and to make him feel the effects of his indignation.

2-: From London and Bristol Delineated, by the English poet and playwright Richard Savage (c. 1698-1743)—as published in The Works of the English Poets. With Prefaces, Biographical and Critical, by Samuel Johnson. Volume the Forty-Fifth (London: Printed by H. Goldney for C. Bathurst, J. Buckland, [&c.], 1779), edited by the English author and lexicographer Samuel Johnson (1709-1784):

In a dark bottom sunk, O Bristol now,
With native malice, lift thy lowering brow!
Then as some hell-born sprite in mortal guise,
Borrows the shape of goodness and belies,
All fair, all smug, to yon proud hall invite,
To feast all strangers ape an air polite!
From Cambria drain’d, or England’s western coast,
Not elegant, yet costly banquets boast!
Revere, or seem the stranger to revere;
Praise, fawn, profess, be all things but sincere;
Insidious now, our bosom-secrets steal,
And these with sly sarcastic sneer reveal.
Present we meet thy sneaking treacherous smiles;
The harmless absent still thy sneer reviles;
Such as in thee all parts superior find,
The sneer that marks the fool and knave combin’d;
When melting pity would afford relief,
The ruthless sneer that insult adds to grief.

3-: From The Foundling. A Comedy. As it is Acted at the Theatre-Royal in Drury-lane (London: Printed for R. Francklin, 1748), by the English playwright Edward Moore (1712-1757):

Bel. Well, Sir!—You have been robb’d you say? [To Villiard.
Vill. And will have Justice, Sir.
Bel. Take it from this Hand then. [Drawing.
Sir Cha. Hold Sir!—This is adding Insult to Injuries.

4-: From On the Subjects Right of Petitioning the King, published in The Political Register, and Impartial Review of New Books (London, England) of December 1769:

It is happy for the king: it is happy for the whole community, that the wisdom and prudence of great and good men have, for the present, checked the alarming resentment of the people, by encouraging them to depend upon the promising hope of success, from a firm, united, and constitutional application to the throne for redress. On the other hand, these writers are daily adding insult to injury, by abusing his majesty’s faithful subjects for their loyalty and moderation, by robbing Billingsgate of its peculiar property, the epithets, scoundrel, and blackguard: and by endeavouring to persuade the petitioners that they have no room for hope, that instead of obtaining relief, they deserve to be punished for presuming to disturb his majesty’s repose.

5-: From Debates of a Political Club, published in The London Magazine: Or, Gentleman’s Monthly Intelligencer (London, England) of June 1770:

Kindness and consideration do not seem the characteristics of our present ministers—For, sir, though they lately received a large supply from the Irish parliament for the purposes of a military augmentation in that kingdom—though they requested this supply in a time of profound peace, and thought it perfectly constitutional to receive it from the representatives of the people; the money was no sooner received, than they denied the right of the commons to grant it, and insisted that power of originating money-bills belonged entirely to the privy council—[…] the Lord Lieutenant having obtained the money, returns thanks to the two houses for their liberality, and after he has politely complimented their munificence, he enters a protest upon the journals of the lords, and sensibly informs the whole world that they were not authorized to exert it—What is this, sir, but adding insult to oppression, but laughing at the idea of all order, and smiling while they stab the essence of all liberty to the heart?

6-: From Debates of a Political Club, published in The London Magazine: Or, Gentleman’s Monthly Intelligencer (London, England) of July 1770:

The great basis of public freedom in this country, most illustrious l—ds, is the constitutional right which the subject enjoys, of electing his representative into parliament. This right has been recently and notoriously violated by the members of the lower club, who, though themselves delegated, presume to appoint their own members, and claim the exercise of a power, even superior to that of the very people from whom alone they derive all their authority. […] If a procedure of this nature is tolerated, what becomes of every franchise which we hold sacred as Englishmen? This is laying the axe at once to the root of our constitution; and arguing in extenuation of it is only adding insult to injury, and desiring us to kiss the hand of the assassin, while he mercilessly plunges a dagger in our hearts.

7-: From Mr. Oliver’s address to the Livery of the city of London, on being elected one of the representatives of the city in parliament, published in The Political Register, and Impartial Review (London, England) of August 1770:

By the means of an enormous, boundless civil list, without account, and a formidable standing army, the ministers of the crown are powerful enough to continue with impunity, to act directly contrary to the united outcries of the subjects, in every part of the British domions [sic], to slight their petitions ill founded, and add insult to contempt.

8-: From Debates of a Political Club, published in The London Magazine: Or, Gentleman’s Monthly Intelligencer (London, England) of September 1770:

Such a set of ministers as the present, so lost to all sense of shame, so eminently above the mere pretence of regard for justice, I never saw. They are not satisfied with trampling upon our rights—they must add insult to oppression; they must make us feel our chains, as well as labour to enslave us.

9-: From On the degeneracy of the Times, published in The Political Register, and Impartial Review (London, England) of November 1770:

Mankind have beheld with astonishment the sacred seat of justice itself prostituted to the direct purposes of tyranny and oppression. They have seen the chief judge of the kingdom, not only refuse to declare the law by which a subject is condemned to perpetual imprisonment, but adding insult to injury, and remanding the prisoner back to confinement, with the sang froid of an officer of the Bastille, saying, He could not help it.

10-: From a speech made by Alderman Townsend during a debate in the House of Commons on 11^th April 1771—as transcribed in The Debates and Proceedings of the British House of Commons, from 1770 to 1772 (London: [s.n.], 1772):

The city of London have an undoubted claim to the soil of the river from Staines-bridge to Yeuland […]. The House acted unjustly in taking away their land without their consent.
They ought to have given a general saving of the rights of the City, but the clause inserted for that purpose is so bad a one, that it adds insult to injury, seeming to offer a trial at law, but in reality giving only a trial; as if the City succeeds, still they cannot have the land contested for.

11-: From a letter from William Bolts to the Court of East India Directors, dated 27^th May 1771—as published in Appendix to Considerations on India Affairs (London: Printed for J. Dodsley, G. Robson, [&c.], 1775), by the Dutch-born British merchant William Bolts (1738-1808):

With respect to the last paragraph of your Secretary’s letter, in which you are pleased to express your sincere wishes and concern for my future conduct, after the injuries I have experienced, I do not know in what light to take it; for I would willingly admit you, gentlemen, to be incapable of adding insult to injury. […] I patiently and silently suffered many injuries in India: yet I never shewed any resentment till I was treated in a manner beyond the power of humanity, witness the proceedings of your Bengal Select Committee of the 18^th and 20^th April 1767 against me, unaccused, unconfronted, and unheard.

12-: From a letter, dated 5^th November 1771, that ‘an American’ wrote to the British Parliament—published in The Boston-Gazette, and Country Journal (Boston, Massachusetts) of 9^th December 1771:

Can posterity believe what the page of truth will record, that “the Britons, regardless of humanity, justice, or gratitude, refused to let the Americans share in the common blessing of freedom; oppressed them with taxes, insulted them with armies, and even shed the blood of some of their citizens!”—And ’tis probable future historians will add, “the Britons continued adding insult to injury, until the Americans, inflamed by such aggravated ingratitude, and unsupportable tyranny, gave vent to their just indignation—destroyed the tyrants, restored liberty, wiped off the reproach of slavery, raised their sullied honour and majesty to the heighth of splendour, and become at once the terror and glory of the world.”