also A1, A-one, "first-rate," 1837 (in Dickens); a figurative use from Lloyd's of London marine insurance company's system for selective rating of merchant vessels ("Register of British and Foreign Shipping"), where it is the designation for ships in first-class condition. The letter refers to the condition of the hull of the ship itself, and the number rating to the equipment. Also used in equivalent ratings in U.S., where colloquially it is sometimes expanded to A No. 1 (which is attested by 1848 as top rating of entries in an agricultural fair).

type of framework shaped like the capital letter "A," by 1889; as a type of building construction in this shape from 1932.

also aha, exclamation of surprise or delighted discovery, late 14c., from ah + ha.

This seely widewe and hire doughtres two ... cryden out "harrow!" and "weloway! A ha! þe fox!" and after him they ran [Chaucer]