Green’s Dictionary of Slang

Sunday (punch) n.

[on the pattern of Sunday, therefore best, suit]
(orig. US)

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.

[[Aus]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].
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.
[US]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.
[US]Pittsburgh Courier (PA) 26 June 20/1: Some fella took a swell ‘Sunday’ on Wynonie Harris.
[US]B. Schulberg Harder They Fall (1971) 103: Even your Luis Firpo was a bum. All he had was a Sunday punch.
[US]‘Ed Lacy’ Lead With Your Left (1958) 37: You... little... bastard... Sunday-punched... me.
[US]H.S. Thompson letter 9 Apr. in Proud Highway (1997) 503: People he knew to be mean drinkers, sunday-punchers and chain whippers.
[US]N. Heard Howard Street 20: He’d pick a fight with the man – usually by throwing a Sunday punch.
[SA](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.

In phrases

cop a Sunday (on) (v.)

(US) to hit someone very hard.

[US]L.A. Times 26 Sept. A7/7: It is likely that Buddy will be out for the rest of the season, for, according to Angel players, the Solon skipper copped a ‘Sunday’ on Van Grafian while the arbiter had his back turned.
[US]A.J. Pollock Und. Speaks 26/1: Cop a sunday, to assault from behind.
[US]P. Kendall Dict. Service Sl. n.p.: cop a Sunday ....sock somebody without warning.
[US](con. 1937) C. Chessman Cell 2455 91: He ran the company. He ran it without ‘copping Sundays,’ catching anyone who attempted to escape or telling the man anything.
[US]F. Elli Riot (1967) 94: The half-wit copped a Sunday on me.
[US]M. Braly False Starts 46: The first he knew he was in a fight was when someone copped a Sunday.