Source : Webster's Revised Unabridged Dictionary (1913)

Underwing \Un"der*wing`\, n.
   1. (Zo["o]l.) One of the posterior wings of an insect.

   2. (Zo["o]l.) Any one of numerous species of noctuid moths
      belonging to {Catocala} and allied genera, in which the
      hind wings are banded with red and black or other
      conspicuous colors. Many of the species are called {red
      underwing}.

Source : WordNet®

underwing
     n : moth having dull forewings and brightly colored hind wings