The Collaborative International Dictionary of English v.0.48:

Pinafore \Pin"a*fore`\, n. [Pin + afore.]
   An apron for a child to protect the front part of dress; a
   tier.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pinafore
    n 1: a sleeveless dress resembling an apron; worn over other
         clothing [syn: jumper, pinafore, pinny]