The Collaborative International Dictionary of English v.0.48:

Antepenultimate \An`te*pe*nult"i*mate\, a.
   Of or pertaining to the last syllable but two. -- n. The
   antepenult.
   [1913 Webster]

WordNet (r) 3.0 (2006):

antepenultimate
    adj 1: third from last
    n 1: the 3rd syllable of a word counting back from the end [syn:
         antepenult, antepenultima, antepenultimate]