Green’s Dictionary of Slang

a very hard or knockout blow; thus as v., to hit with such a blow; thus Sunday-puncher, one who hits with such a blow.

[1889		Dead Bird (Sydney) 2 Nov. 4/3: And he [i.e. a donkey] gave her ‘one for Sunday’ as she struggled on the grass, / Which displaced her summer raiment, and annoyed the happy lass].
1915		D. Runyon in Wash. Post 23 May (Sporting Section) 3/3: I boxed ’im one night, and I hit ’im ’ith my Sunday punch right in the puss, and it didden do no good.
1935		E. Hemingway letter 4 June in Baker Sel. Letters (1981) 414: Landed sunday punch making him hit ass and head almost same time on planks.
1943		Pittsburgh Courier (PA) 26 June 20/1: Some fella took a swell ‘Sunday’ on Wynonie Harris.
1947		B. Schulberg Harder They Fall (1971) 103: Even your Luis Firpo was a bum. All he had was a Sunday punch.
1955		B. Appel Tough Guy [ebook] I’ll take all the Sunday punches you can deliver.
1957		‘Ed Lacy’ Lead With Your Left (1958) 37: You... little... bastard... Sunday-punched... me.
1965		H.S. Thompson letter 9 Apr. in Proud Highway (1997) 503: People he knew to be mean drinkers, sunday-punchers and chain whippers.
1968		N. Heard Howard Street 20: He’d pick a fight with the man – usually by throwing a Sunday punch.
1977		N.C. Heard When Shadows Fall 203: His conscience hit him with the stunning effect of a Sunday punch.
1987		(con. 1950s) G. Moloi My Life 112: Young Election and the rest of the gang then chased the people around the dance floor, driving them towards the door, where Bular and Left tested their so-called ‘Sunday punches’ on the innocent patrons.