[A humble request: If you can, please donate to help me carry on tracing word histories. Thank you.]

 

The dated British-English expression latter wit designates wisdom, a witty remark, etc., which occurs to a person after the event, typically too late to be of use.
—Synonym: esprit d’escalier.

These are, in chronological order, the earliest occurrences of the expression latter wit that I have found:
—Note: With the exception of one mid-18^th-century occurrence (cf. quotation 1 below), these early occurrences are found in texts from—or are associated with—Yorkshire, a county of north-eastern England, on the North Sea, and Lancashire, a county of north-western England, on the Irish Sea:

1-: From The Rambler (London, England) of Tuesday 17^th July 1750, by the English author Samuel Johnson (1709-1784)—as published in The Scots Magazine (Edinburgh, Midlothian, Scotland) of July 1750 [page 306, column 1]—however, here, Latterwit is the name of a character:

I had now visited all my tenants, surveyed all my land, and repaired the old house, which for some years had been running to decay. These proofs of pecuniary wisdom began to recommend me, as a sober, judicious, thriving Gentleman, to all my graver neighbours of the country; who never failed to mention me with honour, in opposition to Thriftless and Latterwit, two smart fellows, who had estates in the same part of the kingdom, which they visited now and then in a frolic, to take up their rents before-hand, debauch a milk-maid, make a feast for the village, and tell stories of their own intrigues, and then rode post back to town to spend their money.

2-: From The Wolves and the Bear. A Fable, dated Monday 19^th July 1852, by ‘N.’, published in the Huddersfield and Holmfirth Examiner (Huddersfield, Yorkshire, England) of Saturday 24^th July 1852 [page 6, column 1]:

While about their loss they griev’d,
They, with that unavailing thought
Called “latter wit,” too often b[o]ught
By men as well as wolves, at rates
For which its fruit scarce compensates,
Own’d that fools only disagree
Where safety dwells with unity;
And, that the loss they underwent,
Was their appropriate punishment.

3-: From a letter to the Editor, by one Joseph Johnstone, published in The Bradford Daily Telegraph (Bradford, Yorkshire, England) of Friday 11^th December 1868 [page 1, column 3]:

Here comes the great smasher for the plaintiff “He has often bragged (they say) about having £200 in the bank.” How this would have floored poor Dobby, providing these sage four had had presence of mind to have mentioned it in court! Oh! latterwit, what art thou doing to miss stating such a pithy bit as this? How the indignation of the court would have been aroused by such a startling revelation!

4-: From the Bury Times, and Heywood, Radcliffe, Ramsbottom, Haslingden, Rawtenstall, & Bacup Advertiser (Bury, Lancashire, England) of Saturday 8^th July 1871 [page 5, column 3]:

“Instructions to the inhabitants of the borough of Bury as to the means for preventing the spread of small-pox and scarlet fever” is the title of a short pamphlet just issued, the author of which is Dr. Bott, Medical Officer of Health under the Bury Improvement Act. […] “Better late than never” is a very venerable saying, but we are inclined to think that if such instructions as the foregoing had been circulated in the town at an earlier period it would have been a good deal better. We are not aware that at present the two particular maladies to which reference is more especially made have assumed anything beyond the average proportions, and to ordinary observers it may seem a trifle singular, and demonstrative of our national faculty of “latter-wit,” that considerably subsequent to the deadly epidemic that caused this union to become so notorious in the mortality returns, a pamphlet of such importance as the one in question should be first brought to light.

5-: From Bakers’ Bothers, an account of a meeting of British bakers, published in The Star (Saint Peter Port, Guernsey) of Thursday 10^th November 1887 [page 4, column 2]:

A Brighton master with a constitution fit to bear the hardest cake e’er baked, or softest dough, suggested that bread was intended to be eaten “like a pudding—as soon as it’s made.” To let a loaf cool before it’s knifed, he added, was a barbarism to be equalled only by the heathen who let a glass of beer fall flat before he drunk it. A latter-wit Yorkshireman ventured to cry out, “An undistributed middle!” but his plea was drowned in applause.

6-: From Raand Abaat th’ Taan, by Sammywell Grimes (i.e., Samuel Grimes), published in Yorkshire Tales. Amusing Sketches of Yorkshire Life. In the Yorkshire Dialect. Second Series (London: W. Nicholson & Sons, [1890?] [chapter 2, page 52]:

It set me thinkin—aw allus do begin thinkin when it’s too lat, for awm like mooast Yorksher fowk, awm troubled wi’ latter wit—but aw’ll keep mi thowts to mysen.