means The natural process by which an animal sheds an outer layer — feathers, skin, shell, or hair — to make way for new growth.

from From Middle English 'mouten,' itself from Old English 'mūtian,' meaning to change or exchange — borrowed from Latin 'mūtāre,' 'to change' (the same root that gives us 'mutate' and 'mutation'). The intrusive 'l' is a later spelling quirk, slipped in by writers who mistakenly thought it belonged there, much like the 'l' that crept into 'fault' and 'could.' So at heart, to molt is simply to change — the creature trading an old self for a new one.