Green’s Dictionary of Slang

A-1 adv.

also A-number-one
[A-1 adj.]

1. excellently, in the best way.

[UK]Leics. Chron. 24 May 12/4: He can patter A1, and get a lot of money.
[UK] ‘’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.
[UK]A. Binstead Houndsditch Day by Day 168: There’s everythink done up A1.
[UK]W.S. Walker Zealandia’s Guerdon 52: My old woman can cook ’em proper, A1.
[UK]E. Pugh Spoilers 133: Your boy’s doin’ A 1, in spite o’ the luggage in his weskit.
[UK]P. Macgill Amateur Army 112: We’ve been at it since eight o’clock, and getting along A1.
[Ire]Joyce Ulysses 102: Everything went off A 1, he said.

2. totally, completely.

[US]G.V. Higgins Cogan’s Trade (1975) 98: He’s fuckin’-A-number-one stupid.
[UK]J. Cameron Vinnie Got Blown Away 64: Got to be A1 mental to do that deliberate.
[UK]J. Hawes Dead Long Enough 72: It’s fucking A1 dismal.