The phrase to nail one’s colours to the mast and its variants mean:
– to make one’s beliefs or intentions plain;
– to stick to or be committed to a plan, opinion, policy, etc.

This phrase alludes to the former practice of nailing an ensign to the mast of a ship, after damage during battle resulted in the ship’s colours no longer being clearly displayed, which otherwise might have been interpreted as a signal of surrender.—These are two mentions of this practice:

1-: From the Foudroyant’s Journal, giving an account of the capture, in March 1800, of the French ship the Guillaume Tell by three British ships, the Penelope, the Lion and the Foudroyant—account published in The Dispatches and Letters of Vice Admiral Lord Viscount Nelson (London: Henry Colburn, 1845):

“Sunday, 30^th March.—[…] At day-break, having all sail set, saw his Majesty’s Ships, Lion and Penelope, engaging a French Line-of-Battle Ship […]. At 6, came up with her, […] and a broadside was fired from her, which was immediately returned within half pistol-shot. Her first broadside cut our rigging very much, and second carried away our fore-topmast and main-topsail-yard. Half past 6, shot away the main and mizen-masts: saw a man nail the French ensign to the stump of the mizen-mast. At 7, Penelope fired at the enemy in passing under her stern. […] 5 minutes past 8, shot away the enemy’s fore-mast. 10 minutes past 8, all her masts being gone by the board, the enemy struck his colours, and ceased firing. Sent boat on board her.”

2-: From The St. James’s Chronicle: Or, British Evening-Post (London, England) of Tuesday 4^th August 1807:

Two more American ships have been detained, and sent into Plymouth. One of these sailed from Philadelphia on the 2d ult. The private letters by this conveyance state, that Commodore Barron of the Chesapeak frigate, has been suspended from his command for allowing himself to be captured.—“You ought,” say the Americans, “to have nailed your colours to the mast, and have fought whilst a timber remained in your ship.”

The Irish playwright and politician Richard Brinsley Sheridan (1751-1816), then Member of Parliament for Stafford, used the metaphor in a speech that he made at the House of Commons on Monday 1^st December 1800, about “the dispute between this Country and the Northern Powers”—speech transcribed in The Observer (London, England) of Sunday 7^th December 1800:

“If that dispute, Sir, arises solely from the assertion of the right of British cruizers to search the vessels of neutral powers, and from our having treated as enemies those vessels which refused to acknowledge this right, I will say that this has been for two centuries the allowed and hereditary privilege of the British Navy; that it is what has rendered it at once the envy and the terror of the whole world; that it is the charter of our power, of our commerce, and of our national greatness. It is the flag which should be wrapped round the body of the State in every hour of danger. It is the colours, which, to allude to the common practice of our brave sailors in action, should be nailed to the masts of the Nation, with which we ought to conquer or to sink.”

The earliest known figurative use of the phrase is from Marmion; A Tale of Flodden Field (Edinburgh: Printed by J. Ballantyne and Co. for Archibald Constable and Company […] ; and William Miller […], and John Murray, London, 1808), by the Scottish novelist and poet Walter Scott (1771-1832):

Record, that Fox a Briton died!
When Europe crouched to France’s yoke,
And Austria bent, and Prussia broke,
And the firm Russian’s purpose brave
Was bartered by a timorous slave,
Even then dishonour’s peace he spurned,
The sullied olive-branch returned,
Stood for his country’s glory fast,
And nailed her colours to the mast.

The following is from a comment on the results of the general election, published in The Manchester Mercury, and Harrop’s General Advertiser (Manchester, Lancashire, England) of Tuesday 27^th October 1812:

The pink and the white, fit emblems of the faint sickly politics of the party that chose them for their insignia, have faded away as quickly as when painted on the flower of spring, while the masculine colours of the “meteor flag of England” of our national standards, have daily acquired a wider glow, and waved to the honour of men who we trust will never dishonour them. Let the emblems of their party put them in mind of their duty, and sooner than give up our maritime rights to France and America, let them nail their colours to the mast head and sink with them.

Thomas Peabody Grosvenor (1778-1817), a Representative from New York, used the phrase in a speech that he made at the House of Representatives, Washington, D.C.—as transcribed in The Pittsburgh Gazette (Pittsburgh, Pennsylvania) of Friday 16^th July 1813:

The American people, even that party which has nailed its colours to the mast of the Executive, and resolved, with him, to go to the bottom, shall no longer doubt the weakness of the President.

The following is from the account of the Sixth Anniversary of the Suffolk Auxiliary Bible Society, held at Ipswich—account published in The Suffolk Chronicle; Or, Ipswich General Advertiser, and County Express (Ipswich, Suffolk, England) of Saturday 25^th October 1817:

The Rev. Mr. Bloomfield.—[…] The lapse of ages might wear down the everlasting mountains—the sun might cease to shed his light upon dissolving worlds—the weary wheels of nature might cease to move—but never could the benefits of the British and Foreign Bible Society cease to be felt throughout the extended universe.—We regarded not as enemies those who thought not with us: we had greater enemies than they—sin and Satan; but we had nailed our colours to the mast, and never would we cease from our endeavours—never would we relinquish the contest—till every country under heaven should participate in the blessed truths of the Gospel.