Green’s Dictionary of Slang

A-1 n.

[A-1 adj. (1)]

someone or something of the first class.

[UK]R.S. Surtees Jorrocks Jaunts (1874) 267: To be surrounded by one’s friends is in my mind the ‘Al’ of ’uman ’appiness.
[UK]R.F. Walond Paddiana I 124: He naturally looked upon himself as the ‘A 1’ of the place.
[UK](con. 1840s–50s) H. Mayhew London Labour and London Poor III 191/2: Juba was first class – a regular A 1 – he was a regular black, and a splendid dancer in boots.
[UK]Leaves from Diary of Celebrated Burglar 108/2: Ned being an inveterate ‘lushington’ [he] soon got ‘turned up’ by the A. 1. No. 1’s, and necessarily became a trudger in the rear of a third-rate ‘duffer’.
O.W. Holmes Poet at the Breakfast Table 298: Where did he get those expressions ‘A1’ and ‘prime’ and so on?