Source : Webster's Revised Unabridged Dictionary (1913)

Mentum \Men"tum\, n. [L., chin.] (Zo["o]l.)
   The front median plate of the labium in insects. See
   {Labium}.

Source : WordNet®

mentum
     n 1: a projection like a chin formed by the sepals and base of
          the column in some orchids
     2: the protruding part of the lower jaw [syn: {chin}]
     3: a projection below the mouth of certain mollusks that
        resembles a chin