Green’s Dictionary of Slang

tough guy n.

[tough adj. (3)/tough adj. (2) + guy n.2 (1)]

1. (orig. US, also tough monkey) a thug.

[US]E.E. Cummings Enormous Room (1928) 195: A terribly tall, rather round-shouldered individual, of whom I said to myself immediately: ‘’By Jove, here’s a tough guy and a murderer all in one.’.
[US]W.R. Burnett Iron Man 30: Coke Mason is just another newspaper tough guy. I could lick him myself.
[UK]V. Davis Phenomena in Crime 204: Plentifully stocked with battalions of ‘tough guys.’.
[US]R. Prather Scrambled Yeggs 70: Tell Cookie there’s a tough guy, a gentleman named Sheldon Scott, wants to see him.
[US]J. Breslin World of Jimmy Breslin (1968) 9: He’s yellow. Put the bull on these tough guys and they all bend in half.
[Can]R. Caron Go-Boy! 36: Fellow prisoners will classify him as a wheel, a solid guy, a tough guy [...] or, if he is thought to be an informer, a rat.
[US]E. Leonard Glitz 113: Kind of a tough monkey, huh?
[Aus]M.B. ‘Chopper’ Read Chopper From The Inside 13: All I ever learnt as a child and teenager was violence and hatred for would-be tough guys.
[US](con. 1973) C. Stella Johnny Porno 40: You wanna cat like a tough guy, save it for the street.

2. someone who cannot easily be checked or thwarted; note use as v. in cit. 1999.

[UK]B. Bennett ‘A Tale of the Rockies’ in Billy Bennett’s Third Budget 25: Jim was the tough guy of Texas, / Quick on the draw was Jim.
[UK]G. Kersh They Die with Their Boots Clean 29: Galileo would have been a Tough Guy to Barker because he couldn’t find it in himself to deny that the earth revolved around the sun.
[US]E. Hunter Blackboard Jungle 98: Well, we’ll see about our good friend Daddy-oh, the tough guy.
[SA]L.F. Freed Crime in S. Afr. 93: The activities of of these juvenile ‘tough guys’ [...] include theft, beatings and a number of vicious pranks.
[UK]B.S. Johnson All Bull 92: He was a tough guy in the real sense of the word.
[UK]J. Sullivan ‘Strained Relations’ Only Fools and Horses [TV script] I’ve always played the tough guy!
[US]J. Ridley Love Is a Racket 231: I big-talked and tough-guyed my way through the day.