Green’s Dictionary of Slang

1. excellently, in the best way.

1855		Broadway Belle (NY) 26 Feb. n.p.: If such is the case, she can brave the storm and come off A. No. 1.
1863		A. Doten Journals 1849-1903 (1973) 24 Dec. 754: [The] best ball I ever saw in my life, I think—everything went off A No 1.
1884		Leics. Chron. 24 May 12/4: He can patter A1, and get a lot of money.
1885		‘’Arry on the Merry Month of May’ in Punch 16 May 229/1: Covent Garden licks Eden, I reckon, at least it’ll do me A 1.
1899		A. Binstead Houndsditch Day by Day 168: There’s everythink done up A1.
1902		W.S. Walker Zealandia’s Guerdon 52: My old woman can cook ’em proper, A1.
1906		E. Pugh Spoilers 133: Your boy’s doin’ A 1, in spite o’ the luggage in his weskit.
1915		P. Macgill Amateur Army 112: We’ve been at it since eight o’clock, and getting along A1.
1922		Joyce Ulysses 102: Everything went off A 1, he said.

2. totally, completely.

1943		W.R. Burnett Nobody Lives for Ever 219: ‘This friend of mine, he’ll get us across the border with no trouble at all [...] None of them [i.. fugitives] ever had any trouble [...] he looks after them A-one’.
1974		G.V. Higgins Cogan’s Trade (1975) 98: He’s fuckin’-A-number-one stupid.
1995		J. Cameron Vinnie Got Blown Away 64: Got to be A1 mental to do that deliberate.
2000		J. Hawes Dead Long Enough 72: It’s fucking A1 dismal.