The Collaborative International Dictionary of English v.0.48:

A 1 \A 1\ ([=a] w[u^]n).
   A registry mark given by underwriters (as at Lloyd's) to
   ships in first-class condition. Inferior grades are indicated
   by A 2 and A 3.
   [1913 Webster]

   Note: A 1 is also applied colloquially to other things to
         imply superiority; prime; first-class; first-rate.
         [1913 Webster]