Green’s Dictionary of Slang

1. a surprise punch, also in fig. use (see cite 2001).

1917		Boston Post (MA) 15 Apr. 18/1: ‘That fellow Dundee is going to get caught with a sucker punch by some strong kid coming off the ropes’.
1919		Eve. Public Ledger (Phila., PA) 9 Dec. 9/6: Many of the spectators did not see the blow that laid the West Philadelphian low. It was a ‘sucker punch,’ as Nick Hayes would have it. [...] Murphy lifted his arms, crossing his right simultaneously, and zowie-- it was curtains.
1947		J.W. Arnold ‘The Lang. of Delinquent Boys’ in AS XXII:2 Apr. 122: Sucker punch. A hit or punch delivered without warning.
1948		R.L. Bellem ‘Death Ends the Scene’ Hollywood Detective May 🌐 It was a sucker punch. It’s always a sucker punch if you lead with your right—unless you’re left-handed.
1955		B. Schulberg On the Waterfront (1964) 313: I guess I did set myself up for a sucker punch.
1975		E. Torres Carlito’s Way 48: Shea threw me a sucker right hand which I caught on my chin—I laid down.
1986		C. Stroud Close Pursuit (1988) 128: A Bronx dealer who had ducked his head forward just as Kennedy had thrown a sucker punch at him.
1988		D. Woodrell Muscle for the Wing 130: Tip, with his variety of sucker punches and his horseshoe-tapped army boots.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 181/1: sucker punch n. 1 an unexpected punch, e.g. an inmate may be sitting down resting and another inmate walks up and punches him on the jaw 2 (from sense 1) any action that takes one by surprise.
2013		T. Robinson Hard Bounce [ebook] [K]eeping my peripheral sharp for any run-up sucker punches.
2021		C. Hammer Opal Country 197: Some arsehole crept up from behind and laid waste to him. A sucker punch.

2. in fig. use of sense 1.

2013		K. Sampson Killing Pool 177: ‘But I did get a printout of the sender’s number, sir.’ She does love her sucker punches, this girl.
2013		T. Robinson Hard Bounce [ebook] [T]he cries of kids who just got a sucker-punch of a reminder that they weren’t immortal.