Source : Webster's Revised Unabridged Dictionary (1913)

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.

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