The Collaborative International Dictionary of English v.0.48:

Armet \Arm"et\, n. [F., dim. of arme arm, or corrupted for
   healmet helmet.]
   A kind of helmet worn in the 14th, 15th, and 16th centuries.
   [1913 Webster]

WordNet (r) 3.0 (2006):

armet
    n 1: a medieval helmet with a visor and a neck guard