Green’s Dictionary of Slang

(US) a tough man, esp. a boxer.

[1899		F. Dumont Witmark Amateur Minstrel Guide and Burnt Cork Encyc. 87: What’s the hardest thing to beat?’ ‘A hard boiled egg.’].
1906		O. Kildare My Old Bailiwick 311: Now don’t be talking like a hard-boiled egg.
[1908		T.A. Dorgan in Zwilling TAD Lex. (1993) 43: Johnny Marco, a real good lad, is to battle Young Laughrey, the ‘Hard Boiled Egg’ from Philadelphia. Hard boiled egg. Can’t be beat].
1916		T.A. Dorgan Indoor Sports 8 Feb. [synd. cartoon] He may be some people with the ladies but heeza hard-boiled egg around the boys.
1923		H.C. Witwer Fighting Blood 24: Knocking around since I been a kid has made me a pretty hardboiled egg.
1931		M. Harris ‘Facing the Mob’ in Gangland Stories Feb. 🌐 He’s a thirty-minute egg.
1932		R.E. Howard ‘Night of Battle’ in Fight Stories Mar. 🌐 Bad Bill was a hard-boiled egg which run a gambling hall in the toughest waterfront district of Singapore.
1962		Mad mag. Mar. 47: TV Dinners for [...] Private Eye Shows: Hard-Boiled Eggs. Hot Tomatoes. Assorted Capers.
1967		Wodehouse [title] Jeeves and the Hard-Boiled Egg.