Put the Tin Hat on it

[airbolt]

What are the origins of the phrase "That's put the Tin Hat on it"? I've always taken it to mean something similar to " That's the final straw" - in other words the final step in a disaster.

[Quizmonster]

Tin hat was the slang name soldiers gave to the steel helmet they wore to protect them from shrapnel etc at the start of the 20th century. It was, I imagine the last bit of kit they put on when getting into uniform and heading into action. Hence, the idea that "putting the tin hat on" something was, in effect the last straw.
Lid is also a colloquial name for a hat and the phrase "putting the lid on " something meant much the same thing
The earliest recorded use of the hat phrase in this way, however, dates back to just after World War I.

[juandelacruz]

I'd agree with airbolt mostly. "Put the lid on it" dates back to the 19th century and is more likely to mean the lid on a box or whatever. Eric Partridge (Dictionary of Catch Phrases) says the tin hat variation is Liverpool.