Green’s Dictionary of Slang

tin hat n.1

1. [20C+] (also tin derby, tin lid) a helmet.

2. [1910s] a senior officer [rare var. on brass hat under brass adj.1 ].

In phrases

put the tin hat on (v.) (also put the brass hat on, ...tin lid on) [WWI milit. tin hat, a steel helmet]

[20C+] to perform an ultimate, usu. negative, act, to finish (off) completely.