The phrase to have a shot in the locker means:
– to have something in reserve, but ready for use;
– (sometimes specifically): to have money in reserve;
– to have a chance or opportunity remaining.

A variant of the phrase to have a shot in the locker occurs, for example, in No rate cuts for now—but beware the baying of stagflation, by Russell Lynch, published in the London Evening Standard (London, England) of Thursday 27^th October 2016—the author explains why consumers must not expect “an early Christmas present from the Bank of England next week in the form of an interest rate cut”:

It’s adding up to a mild case of “stagflation”; slowing growth and rising inflation, which promises a tricky spell for the first half of next year. It won’t be as bad as 2008, which saw inflation above 5% and a sharp recession, but still a cold shower for the consumers our economy relies so much on. All the more reason, then, for the Bank to leave one final shot in its locker on rates until the new year.

In the phrase to have a shot in the locker, which is of nautical origin:
– the noun shot denotes a projectile designed for discharge from a firearm;
– the noun locker denotes the compartment for keeping ammunition on a ship.

The earliest occurrences of the phrase to have a shot in the locker that I have found are as follows, in chronological order:

1-: From The Independent Gazetteer; Or, The Chronicle of Freedom (Philadelphia, Pennsylvania, USA) of Tuesday 23^rd June 1789:

EPITAPH,

Offered for the Monument of the Honorable Job Pray, who at the time of his decease, was Member of the Honorable the Executive Council of this state; and during the late war, a naval commander in the service of the United States of America.

Sunk at his moorings,
On Wednesday the 29^th of Apirl [sic],
M,DCC,LXXXIX,
One, who never struck his flag ¹,
While he had
A Shot in the Locker;—
Who carried sail, in chace,
Till all was blue;—
[&c.]

¹ The phrase to strike one’s flag means to surrender.

2-: From Bannian Day, a musical entertainment, in two acts, performed at the Theatre Royal, Hay-Market (London: Printed for T. N. Longman, 1796), by the English author George Brewer (1766-?):

Polly. Lord, Jack! what’s the matter?
Jack. All a-back, Polly; a-ground on the worst of all shores, the rocks of misfortune.
Polly. Lord! what’s the matter? what has happen’d?
Jack. Nothing to myself, Polly; I’ve always got a shot in the locker, just enough to keep Davy Jones ² out of my hammock, as you may say; my heart’s too light to be sunk by my own misfortunes, but when another’s are in tow, all my spirits go to the bottom at once.
Polly. Well; but what is it you want?
Jack. The rhino ³, Polly.

² Davy Jones designates the spirit or devil of the sea.
³ The noun rhino is slang for money.

3-: From Bill Bobstay, by the English actor, composer and author Charles Dibdin (bap. 1745-d. 1814), published in The Yorkshire Musical Miscellany; Comprising an elegant selection of the most admired songs in the English language, set to music (Halifax: Printed by C. Jacobs, 1800):

My bark, tho’ hard storms on life’s ocean should rock her,
Tho’ she roll in misfortune, and pitch end for end,
No, never shall Bill keep a shot in the locker,
When by handing it out he can succour a friend.

4-: From The Ship Constitution, a song “sung at the celebration of American independence, the 4^th inst. at Stockbridge, composed for the occasion”, published in The Spectator (New York City, New York, USA) of Wednesday 5^th August 1801:

But cheerly, my lads! we have been on the ocean,
And rode out the puffers, full many’s the day;
And stiff must the gales blow, and dire the commotion
That baffle our ship, or our bosoms dismay;
Tho’ foes should assail, or the tempest should rock her,
Tho’ white squalls of faction should take her abeam,
Here’s stand by her stuff while a shot’s in the locker,
And cling to her side while she floats on the stream.

5-: From A House to be Sold, a musical piece, in two acts. As performed at the Theatre-Royal, Drury-Lane (London: Printed for G. and J. Robinson, 1802), by the English librettist James Cobb (1756-1818)—the speaker is a young officer in the navy:

Jack, you are a good fellow, and I wish with all my soul it were in my power to offer you any thing. To-morrow, perhaps, I may be in better trim; but at this present speaking, I have not a shot in the locker.

6-: From The True Briton (London, England) of Monday 6^th September 1802:

PALACE COURT, Sept. 4.
CARTERMOLD v. GARDINER.

The Plaintiff in this Cause was a Publican, who kept the Crown public-house in Chandos-street; and the Defendant was a son of Neptune, but as it appeared not altogether impressed with that rigid principle of honesty which in general distinguishes a British Tar. Jack had a girl, but he had not the means of keeping her—Jack loved good eating and drink, but he had no shot in the locker. To satisfy all his wants, he was obliged to have recourse to his wits.

7-: From an untitled ballad by the English actor, composer and author Charles Dibdin (bap. 1745-d. 1814), published in The professional life of Mr. Dibdin, written by himself. Together with the words of six hundred songs selected from his works (London: Published by the author, 1803):

Thanks to fortune at sea I’ve no need to break bulk ⁴,
For I’ve plenty of shot in the locker.

⁴ The phrase to break bulk means to abstract part of the cargo of a ship.

8-: From a song, “written by Mr. Harpley, […] received with much eclat, at the meeting of the Liverpool Marine Society, on the 3d inst.”, published in The Lancaster Gazette; and General Advertiser, for Lancashire, Westmorland, &c. (Lancaster, Lancashire, England) of Saturday 14^th January 1804:

Enraptur’d we’ll hail this our festival day,
The welcome return of our annual treat:
That tribute to pay which supports the blest store,
The comfort, the pleasure, and joy of our lives;
Because it still guards from disaster secure,
A shot in the locker for children and wives.

9-: From The Marine Chaplain. A Discourse Figurative and Political, written for, and published in, the Political Observatory (Walpole, New Hampshire, USA) of Saturday 6^th April 1805:

The mutineers on board ship, under the new commodore (Jefferson) finding that Facts gave the lie to their predictions, became passionately interested to produce the consequence they had so confidently foretold, and when they beheld the vessel proceeding with a full tide and fair breeze, their envy and malignity were summoned to their aid: these dangerous auxiliaries have made a blot in the page of our history which it is painful to review. It will not be necessary, I hope, to detail the falsehoods and misrepresentations, arts and delusions which have for base purposes been resorted to by those agents.
Our vessel set out upon a different tract from her former one: those who had a knowledge of her intended rout [sic], were confident of the propriety of adopting it, but as many on board were unacquainted with the latitude and longitude, their fears, their credulity and their superstition (nearly equal to those of the people who sailed with Columbus) were assailed by every artifice. How often were they solemnly told, that they were fast approaching the shoals of anarchy, or drifting far out of their course by the current of Democracy? Many of those, whose business it is to cast the line, (Printers) have been induced to sing out falsely “shoal water!” when they could not fathom the bottom. Others in higher stations cried out, “a strange sail ahead!” which they insinuated was an Algerine or French pirate. Others below called out “no shot in the locker!” A fourth class sounded a higher note, “Treachery! treachery! our commander is going to sacrifice us to the first pirate who orders us to strike!”