Brewer's: Able-bodied Seaman
A sailor of the first class. A crew is divided into three classes: (1) able seamen, or skilled sailors, termed A.B.; (2) ordinary seamen; and (3) boys, which include green-hands, or inexperienced men, without regard to age or size.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894