"jellyfish," 1758, as genus name, from Medusa, the name of one of the three Gorgons with snakes for hair, whose glance turned to stone whomever looked upon it (attested in English from late 14c.). Her name is from Greek Medousa, literally "guardian," fem. present participle of the verb medein "to protect, rule over" (from PIE root *med- "take appropriate measures"). The zoological name was chosen by Linnæus, suggested by the creature's long tentacles. Related: Medusoid.
before vowels pneum-, word-forming element meaning "lung," from Greek pneumōn "lung," altered (probably by influence of pnein "to breathe") from pleumōn (which was an alternative form in Attic), literally "floater," probably cognate with Latin pulmo "lung(s)," from PIE root *pleu- "to flow." The notion perhaps is from the fact that, when thrown into a pot of water, lungs of a slaughtered animal float, while the heart, liver, etc., do not (compare Middle English lights "the lungs," literally "the light (in weight) organs"). Greek pneumōn also meant "jellyfish, medusa," "perhaps from its rhythmical pulsation, as if breathing" [Thompson].