early 14c., from glow (v.) + worm (n.). Actually the wingless female form of a beetle (Lampyris noctiluca). The males have wings but do not glow.
Entries linking to glow-worm
glow v.
Old English glowan "to glow, shine as if red-hot," from Proto-Germanic *glo- (source also of Old Saxon gloian, Old Frisian gled "glow, blaze," Old Norse gloa, Old High German gluoen, German glühen "to glow, glitter, shine"), from PIE root *ghel- (2) "to shine," with derivatives referring to bright materials and gold. Figuratively from late 14c. Related: Glowed; glowing. Swedish dialectal and Danish glo also have the extended sense "stare, gaze upon," which is found in Middle English.
worm n.
Old English wurm, variant of wyrm "serpent, snake, dragon, reptile," also in later Old English "earthworm," from Proto-Germanic *wurmiz (source also of Old Saxon, Old High German, German wurm, Old Frisian and Dutch worm, Old Norse ormr, Gothic waurms "serpent, worm"), from PIE *wrmi- "worm" (source also of Greek rhomos, Latin vermis "worm," Old Russian vermie "insects," Lithuanian varmas "insect, gnat"), from PIE *wrmi- "worm," from root *wer- (2) "to turn, bend."
The ancient category of these was much more extensive than the modern, scientific, one and included serpents, scorpions, maggots, and the supposed causes of certain diseases. For substitution of -o- for -u-, see come. As an insult meaning "abject, miserable person" it dates from Old English. Worms "any disease arising from the presence of parasitic worms" is from late Old English. Can of worms figurative for "difficult problem" is from 1951, from the literal can of worms a fisherman might bring with him, on the image of something all tangled up.