Green’s Dictionary of Slang

tough-nut adj.

also tough-nuts
[tough nut n.]

aggressive, macho, obstinate.

[US]W. Diehl Sharky’s Machine 245: This tough-nuts shooter you talkin’ about queer?
[US](con. 1949) G. Pelecanos Big Blowdown (1999) 285: Seems he couldn’t help from bein’ his usual tough-nut self.
[UK]Observer Screen 27 June 10: It features [...] Lionel Jeffries (as a tough-nut warder).
[UK]Indep. Rev. 28 Jan. 10: The arrival of Tommy Lee Jones as Judd’s tough-nut parole officer.
[UK]Week 8 Jan. 18/2: the trouble was caused by a group of ‘tough-nut’ prisoners.