'bagworm' originates from English as a compound of 'bag' and 'worm', where 'bag' comes from Middle English 'bagge' (from Old Norse 'baggi') meaning 'bundle' or 'pack', and 'worm' comes from Old English 'wyrm' meaning 'serpent' or 'creeping creature'.

'bag' (Middle English 'bagge', Old Norse 'baggi') + 'worm' (Old English 'wyrm') were combined in English to form the compound 'bagworm' to describe larvae that live in bag-like cases; the modern spelling and usage stabilized in post-medieval English.

Initially the elements meant 'bag' + 'creeping creature'; over time the compound came to refer specifically to moth larvae that inhabit a bag-like case and, by extension, to certain pest species.