Green’s Dictionary of Slang

tough nut n.

[tough adj. (2) + nut n.1 (1b); note SE phr. tough nut to crack]
(orig. US)

1. (also rough nut, tough cud) a person who is difficult, obstinate or dangerous to deal with.

[WI]E.W. Pearson Letters from Port Royal 81: There are a great many men of twenty-five to forty, ‘tough-nuts’ many of them.
[US]J.W. Carr ‘Word-List from Hampstead, N.H.’ in DN III iii 203: tough cud, n. A hard character.
[UK]Gem 17 Oct. 17: To tell you the truth, Merry, he’s a tough nut.
L.M. Staley letter 6 Aug. at [Internet] He is a rather rough nut, but not as bad as some of this crowd.
[US]‘Ellery Queen’ Roman Hat Mystery 221: I know well enough you’re a tough nut, Inspector!
[US]A. Bessie Men in Battle 69: He is real dirty but hard as nails. ‘Great kid,’ the men say, ‘a tough nut.’.
[UK]A. Wright Under the Whip 7: Andrew seems to be quite a tough nut when he gets down to talking business.
[UK]C. MacInnes Absolute Beginners 141: From now on I’m going to be a tough, tough nut, and if she thinks she can hurt me, she’s bloody well mistaken.
Analog Science Fiction/Science Fact 70: ‘You’re looking for trouble, O’Hare — this Farquhar is a tough nut.’ [...] ‘O’Hare is a tough nut, too, and can take care of himself.’.
J.B. Sanford Color of the Air 146: ‘I thought you were a tough nut.’ ‘Oh, I am,’ you’d say. ‘I chew Star Navy and spit.’.
[UK]Indep. 26 Sept. 29: But they are tough nuts all the same.
[UK]Guardian G2 23 Mar. 22: I was tickled by the story of a tough nut [...] who had to be served a summons.
[Aus]S. Maloney Big Ask 56: Big Bob was a legend. A rough-nut among the silvertails. The man they couldn’t root, shoot or electrocute.
[US]D. Winslow Winter of Frankie Machine (2007) 84: Abe doesn’t need to be asked twice. Patty’s going to be a tougher nut.

2. attrib. use of sense 1.

B. Lefkowitz Our Guys 149: Richie, a tough-nut kid. He was better than anything else we had. And he did an admirable job. Above-average toughness.