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) [WWI milit. tin hat, a steel helmet]

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

St Sndrews 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.