Green’s Dictionary of Slang

someone or something of the first class.

1838		R.S. Surtees Jorrocks Jaunts (1874) 267: To be surrounded by one’s friends is in my mind the ‘Al’ of ’uman ’appiness.
1847		R.F. Walond Paddiana I 124: He naturally looked upon himself as the ‘A 1’ of the place.
1850		Manchester Spy (NH) 26 Oct. n.p.: [B]elonging to the class A. No. 1.
1861		(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.
1865		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’.
1872		O.W. Holmes Poet at the Breakfast Table 298: Where did he get those expressions ‘A1’ and ‘prime’ and so on?