Green’s Dictionary of Slang

hard bit n.2

[SE hard + bit n.1 (3b)]

(US prison) an unpleasant time in prison because of one’s personality, or crime (which may alienate other prisoners), or inability to adapt etc.

[US]P. Thomas Down These Mean Streets (1970) 321: I saw myself as I had been six years ago, hustling, whoring, and hating, heading toward the same long years and the hard bit.
[US]R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].
[US](con. 1970s) G. Pelecanos King Suckerman (1998) 88: Those hard bits Ronald had to do later on.