The phrase “A number one” means superior, first class, the best of its kind.
An American nautical classification of British ancestry, both of which referred originally to sailing vessels.
The British term, usually written “A 1,” was thus described in Lloyd’s Register: “The character A denotes new ships, or ships renewed or restored. The stores of vessels are designated by the figures 1 and 2; signifying that the vessel is well and sufficiently found.”
The American term had a slightly different sense, as described in Goodrich’s Fifth School Reader (1857): “Vessels are classified according to their age, strength, and other qualities. The best class is called A, and No. 1 implies that the Swiftsure stands at the head of the best class of vessels.”
Charles Dickens was the earliest writer to give the British phrase a non-nautical use. In Pickwick Papers (1837), the faithful valet, Sam Weller, wants to know what kind of “gen’l’men” already occupy the prison room in which Mr. Pickwick is to be confined.
The turnkey, Roker, describes one who “takes his twelve pints of ale a-day, and never leaves off smoking even at his meals.” “He must be a first-rater,” says Sam. “A-1,” Roker answered.
And Harriet Beecher Stowe, in Dred, A Tale of the Great Dismal Swamp (1856), has the distinction of being the first to introduce the American phrase into literary use.
She has Father Bennie, the preacher who buys and sells slaves as a sideline, ask a dealer, “You got a good cook in your lot, hey?” “Got a prime one,” the dealer answered, “an A number one cook, and no mistake.”