Source : Webster's Revised Unabridged Dictionary (1913)

Pinafore \Pin"a*fore`\, n. [Pin + afore.]
   An apron for a child to protect the front part of dress; a
   tier.

Source : WordNet®

pinafore
     n : a sleeveless dress resembling an apron; worn over other
         clothing [syn: {jumper}, {pinny}]