any of a family (Psychidae) of moths with larvae that build cocoons shaped like bags and feed on the leaves of various trees: the wingless, legless adult females continue to live within the casing
See bagworm in American Heritage Dictionary 4
noun
Any of several moths of the family Psychidae, which construct fibrous cases of silk spun together with leaves, twigs, or grass. The plant-feeding larvae and wingless adult females live in these cases.