also moult, mid-14c., mouten, of feathers, "to be shed," from Old English *mutian "to change" (in bemutian "to exchange"), from Latin mutare "to change" (from PIE root *mei- (1) "to change"). Transitive sense, of birds, "to shed feathers" is first attested 1520s. With unetymological -l-, late 16c., on model of fault, etc. Related: Molted, moulted; molting, moulting. As a noun from 1815.