Green’s Dictionary of Slang

(US) to prepare for a fight, to adopt an aggressive posture; also as n. the moments/position of preparation for a fight.

1830		J.B. Skillman N.Y. Police Reports 35: A square off — roll up sleeves — a regular set-too [sic] — down went the Nigger — up again, as quick as a piece of a sturgeon's nose.
1838		J.C. Neal Charcoal Sketches (1865) 41: If he ‘squares off’ at a big fellow, he is obliged in dealing a facer, to hit his antagonist on the knee.
1848		‘Ned Buntline’ Mysteries and Miseries of N.Y. III 57: He [...] squared off into a pugilistic position for a fight.
1856		‘Q.K. Philander Doesticks’ Plu-ri-bus-tah 126: Then, at once, squared off at Cuffee, / Instantly ‘sailed into’ Cuffee; / And he whaled away at Cuffee.
1872		Wkly Kansas Chief (Troy, KS) 26 Dec. 1/4: ‘Don’t tech me!’ Bill threatened, squaring off.
1893		S. Crane Maggie, a Girl of the Streets (2001) 20: Den deh mug he squared off an’ said he was fine as silk wid his dukes.
1905		G.W. Peck Peck’s Bad Boy Abroad 83: The Englishman squared off like a prize fighter.
1917		Hall & Niles One Man’s War (1929) 230: I thought it was time to go into action, so I squared around for a good scrap.
1928		C. McKay Home to Harlem 308: The girls rushed to the window and saw the two black women squaring off at each other down in the back yard.
1938		J. Weidman I Can Get It For You Wholesale 187: I figured the time wasn’t ripe yet for us really to square off.
1945		F.H. Hubbard Railroad Avenue 199: The boomer squared off and gave the super the once-over.
1955		R. Graziano Somebody Up There Likes Me (1956) 224: Baksi and I squared off ready to tear into each other.
1964		R. Abrahams Deep Down In The Jungle 152: Says, ‘I’m gonna beat you, motherfucker, though you’re twice my size.’ He looked back and squared off to fight.
1972		R. Barrett Lovomaniacs (1973) 326: The gossip is now that Irving and Jackie may square off.
1981		(con. 1966) P. Conroy Lords of Discipline 218: The mere thought of the two of them squaring off for combat filled me with a nameless dread.
1997		(con. 1970s) G. Pelecanos King Suckerman (1998) 223: I’m squarin’ off against Wilton Cooper and his boys tonight.
1999		Guardian Guide 21-27 Aug. 5: Lauren Bacall was all of 17 when she squared off against a 45-year-old Humphrey Bogart.
2000		Guardian G2 28 Feb. 16: If those two square off, the race to put Paris back on the map will be every bit as nasty as London’s has already become.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 175/2: square off v. 3 to prepare for a fight, to fight.
2005		Des Moines Register (IA) Community Forum 15 Feb. 17/6: The Maroons wait as north and Waukee square off.