Green’s Dictionary of Slang

hard-boiled adj.

[hard-boiled egg n.]

1. [20C+] tough, mean, unpleasant.

2. [1900s] (US) used as intensifier.

SE in slang uses

In compounds

hard-boiled collar (n.)

[1920s–50s] a stiff, starched, detachable collar.

hard-boiled hat (n.) (also hard-boiled lid)

[1900s–60s] (US) a stiff hat.

hard-boiled shirt (n.) (also hard-boiled chest protector) [ext. of boiled shirt under boiled adj.]

[1910s–30s] (US) a stiff, starched detachable shirt front.