Yours, Etc.: Origins and Uses of 8 Sign-Offs
also hardhat, hard-hat, late 14c., "helmet;" 1935, "derby hat;" meaning "safety helmet" is from 1953; used figuratively for "construction worker" from 1970.
: Caution, this is a hard-hat zonenoun phrase
[the political sense fr the vocal and sometimes violent opposition of many construction workers to the US peace movement during the Vietnam War, reinforced by terms like hard line and hard core]