Definition of "underwing" [un•der•wing]

• One of a pair of hind wings of an insect, such as a moth. (noun)
• Any of various noctuid moths of the genus Calocala, having brightly colored hind wings visible only during flight. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "It has several minor failings — among them limited a internal weapons carriage, rendering underwing carriage necessary (thus negating most of its stealth advantages), along with an inability to fire its air-to-air weapons at maximum speed."
• "Match the artsy underwing of the pearl-bordered fritillary with the breathtaking views from the Morecambe Bay Limestones, just below the Lake District."
• "I am guessing that this one is called the Modest Sphinx Moth because the plum coloured underwing is generally hidden."