Green’s Dictionary of Slang

tin hat n.1

1. (also tin derby, tin lid) a helmet.

[UK]Eve. Teleg. 13 Oct.2/6: Saved by a Tin Hat [...] He states he was wounded on 1st October by shrapnel [...] ‘I’d had been dead as a doornail [...] had it not been for my tin hat.
‘Wally’ Wallgren [comic strip] Never unnecessarily stop a descending shell with your tin derby.
L.N. Smith Lingo of No Man’s Land 86: TIN HAT Tommy's name for the steel helmet. The name sometimes applied to staff officers.
R.L.Thompson Hist. of 310th Infantry 29: The one way to keep the cap from acting as a sponge was to wear it under a ‘tin lid.’.
[Aus]W.H. Downing Digger Dialects 50: tin hat — Steel helmet.
[Aus](con. WWI) A.G. Pretty Gloss. Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: tin hat. A flat steel helmet for protecting the head against shrapnel bullets.
[UK]Western Dly Press 4 Apr. 12/5: It is not right [...] to take a man from a farm or factory, put a tin hat on him and then, if his nerve fails, shoot him.
[Aus]Western Mail (Perth) 19 Feb. 2/1: They were discussing tin hats at the reunion [...] ‘That’ll tell yer if their tin lids were any good’.
[US]Phila. Eve. Ledger 20 July n.p.: ‘Tin hat’--trench helmet.
[UK]Western Morn. News 3 May 3/6: Used ‘Tin Hat’ as Broom [...] A gas company worker recently found a new use for his steel helmet [etc.].

2. a senior officer [rare var. on brass hat under brass adj.1 ].

see sense 1.
[US]Mencken Amer. Lang. (4th edn) 573: A few novelties came in, e.g., tin-hat and a.w.o.l., and there was some fashioning of counter-words and phrases from French materials.

In phrases

put the tin hat on (v.) (also put the brass hat on, ...tin lid on) [seemingly Boer War coinage with ref. to container lids (rather than assumed World War I and steel helmets) ]

to perform an ultimate, usu. negative, act, to finish (off) completely; note extrapolation in cit. 1970.

Kentish Mercury 3 June 3/6: Their cavalry tried to out-flank us, but our horse and the Maxims ‘put the tin hat on them’ .
[UK]Durham Co. Advertiser 29 Dec. 8/2: LETTER FROM A DURHAM SOLDIER. HOW HE PUT ‘THE TIN HAT’ ON A FLEEING BOER [...] One them tried get away, but did not get far. I soon had a bullet on its way to seek him, and it found his leg and dropped him. That put the ‘tin hat on’ but we had to carry the blessed sod in then. My regards to all home.
[UK]St Andrews Citizen 17 Mar. 3/4: The fighting lasted three [...] days, but the third day put the tin hat on it. We had them on the hill, and [...] subjected them to terrible fire. By night we saw them retiring.
[UK]Hull Dly Mail 24 Dec. 2/4: The poor support when we called a meeting put the ‘tin hat’ on the suggestion.
[UK]Liverpool Echo 15 Feb. 3/7: I am in great demand as an interpreter; but somebody put the tin hat on it the other day by calling me an ‘interrupter!’.
Dly Teleg. (Launceston, Tas.) 17 May 11/2: They revolted, demanded higher pay, and refused to write any more for him until they got it. This totally unlooked for crisis fairly put our author into a corner - put the ‘tin hat’ on him as they would nowadays say - and [...] much against his will, was forced to accede to the demands of the not too highly paid scribes.
[UK]N&Q 12 Ser. IX 467: Tin Hat On It (Just About Put the). Nearly spoilt everything.
[UK](con. WWI) Fraser & Gibbons Soldier and Sailor Words 282: Tin Hat On It, To Put A: To finish. To overdo. To add the last straw.
[UK](con. WWI) ‘Sapper’ Shorty Bill 256: And the combination [of words] put the brass hat on.
[UK]Western Dly Press 19 Mar. 9/7: It really is the Parish Council who have ‘put the tin hat on it’.
[Ire](con. 1880–90s) S. O’Casey I Knock at the Door 182: I’ll dance in then and put the tin hat on him.
[UK]Eve. Teleg. (Dundee) 25 Jan. 3/4: Friday night put the tin hat on Scottish listeners [to the radio].
[UK]H. Pinter Caretaker Act III: And to put the old tin lid on it, you stink from arse-hole to breakfast time.
[UK]L. Dunne Goodbye to The Hill (1966) 167: And to put the tin hat on it there was a fella in the front row who could sing ten times louder than I could.
[UK]A. Sillitoe Start in Life (1979) 87: When his head finally hit the tin lid.
[Ire]P. McCabe Breakfast on Pluto 45: I think what put the tin hat on it was when they decided to top young Laurence Feely.
Twitter 24 Dec. [Internet] The discovery that Scottish 80s funkateer Jesse Rae had a brief & unsuccessful career in porn under the name ‘Jock Pleasure’ has put the tin hat on this fuckin’ year.