'bagworm' originates from the combination of the English words 'bag' and 'worm', where 'bag' referred to a pouch or sack and 'worm' was an old word for a creature (not necessarily a true worm).

'bagworm' developed as a compound in English by joining 'bag' + 'worm' (Old English 'wyrm' for a creeping creature); over time the compound came to specifically denote the case-making moth larvae now called 'bagworm'.

Initially, 'worm' broadly meant any creeping or legless creature; over time the compound came to mean specifically the larval moth that lives in a 'bag', and now 'bagworm' refers either to the insect or to its characteristic case.