Source : Webster's Revised Unabridged Dictionary (1913)

A B C \A B C"\ ([=a] b[=e] s[=e]").
   1. The first three letters of the alphabet, used for the
      whole alphabet.

   2. A primer for teaching the alphabet and first elements of
      reading. [Obs.]

   3. The simplest rudiments of any subject; as, the A B C of
      finance.

   {A B C book}, a primer. --Shak.