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: Webster's Revised Unabridged Dictionary (1913)