Red Underwing Moth.

"This moth is quite huge and is beautifully camouflaged until it reveals those shocking red markings on its hindwings. Its scientific name is Catocala nupta. The genus name Catocala means “beautiful hindwings” and the nupta species alludes to the fact that the kinky naturalists who named it thought the moth had flashy red bloomers that it revealed only on its wedding night"

Text from the excellent

www.sciencebase.com/science-blog/red-underwing.html

which also has a picture showing the flashy red bloomers!