Green’s Dictionary of Slang

to pay off one’s debts (to a person), whether financial or otherwise; thus squaring-up n., the settlement of debts; squared-up adj., paid off.

1858		R.S. Surtees Ask Mamma 291: ‘Ve vill square it op some other day. Here,’ added he to the damsel, ‘dis gentleman vill pay, and I vill settle vid him some oder day.’.
1865		Leaves from Diary of Celebrated Burglar 90/2: We then finished the bottle, and walked to the bar where, on ‘squaring up’ with Ned, he said he hoped all was ‘cocum’.
1871		C. Money Knocking About in N.Z. 16: Having ‘squared up,’ we struck the tents.
1884		Sporting Times 25 Oct. 2/2: ‘He will offer to square up for him [...] I daresay he owes Sam a bit’.
1888		‘Rolf Boldrewood’ Robbery Under Arms (1922) 123: I’ll be there tomorrow and square up.
1895		Crowe Aus. Sl. Dict. 79: Square Up, settle up.
1900		Bulletin (Sydney) 17 Mar. 13/2: at the squaring-up. / The Detective: ‘Look at the beers I bought yer.’ / The Phizgig: ‘But look at the dirt yer made me swaller!’.
1903		J. Furphy Such is Life 9: If the mean beggard’ on’y square up with me. [Ibid.] 184: I want to be kep’ at my own proper work, or else sacked an’ squared-up with.
1916		J. Lait ‘The Gangster’s Elegy’ in Beef, Iron and Wine (1917) 247: There ain’t a lad in our end o’ the graft what wouldn’ take a chance after that bank roll an’ to square up for what them bottom-dealers done the Kid.
1923		H.C. Witwer Fighting Blood 243: Well, we’re all squared up, then [...] Would you mind giving me a receipt – eh – say: ‘Received of Maurice Dempster ten thousand dollars.’ Sort of make it – eh – regular?
1936		D. Stivens Tramp and Other Stories 139: Neil borrowed a shilling from me some time ago—and forgot to pay it back. [...] Just to square things up I borrowed a shilling from him and forgot to pay it back.
1940		O. Strange Sudden Takes the Trail 109: Next time yu heave anybody out’n that grub palace let him square up first.
1952		W. Sheldon Troubling of a Star 120: Take care o’ the bill, will you, ol’ buddy? I’ll square up with you later.
1967		J. Breslin World of Jimmy Breslin (1968) 36: If you’re doing any good [...] we’ll square up.
1979		‘Iceberg Slim’ Airtight Willie and Me 119: You heard those Hilsons swear to hit us if we don’t square up with them.
1982		G. Newbold Big Huey 16: I had two good men to whom I could front the gear for $7.50, and who would always square me up at the end of the week.
1993		I. Welsh Trainspotting 86: I’d [...] tell the poor bastard thit ah’d already squared the cunt up.
1998		P. Doyle (con. late 1950s) Amaze Your Friends (2019) 15: [A]ssurances from the band that, too right, they’d square up with me [...] no problem.
2006		B. Matthews Intractable [ebook] ‘Bernie, how do you suggest I should square up?’.