Douglas Harper's Etymology Dictionary
frass
insect excrement, 1854, from German frasz, from root of fressen "to devour, to eat as a beast does" (see fret (v.)).
Wiktionary
frass
n. The droppings or excrement of insects.
Wikipedia
Frass
Frass refers loosely to the more or less solid excreta of insects, and to certain other related matter.