Green’s Dictionary of Slang

1. a tough man or boy.

1947		M.H. Boulware Jive and Sl.
1966		Current Sl. I:3 4/1: Hard leg, n. Boy. [...] There ain’t nothing but hard legs at that party.
1970		A. Young Snakes (1971) 113: She was sitting on the sofa next to some curly-headed hardleg.
1985		T.R. Houser Central Sl. 27: hard legs A guy [...] ‘Ain’t no girls in the house, just a bunch a hard legs.’.

2. a man who devotes all his time and energies to pursuing the street life and the world of strictly male endeavour – pimping, hustling etc.

1983		‘Black English’ in Reader’s Digest Success with Words 86/1: hard leg = (1) ‘a man’.
1983		R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].

3. an ugly woman, esp. an old, worn-out prostitute.

1969		‘Iceberg Slim’ Pimp 177: She ain’t no ‘hard-leg’.
1971		Winick & Kinsie Lively Commerce 43: The old whore is a ‘hard leg’—she’s got a million miles on her.