jellyfish

Jellyfish combines jelly and fish. Jelly traces back through Old French gelee and Latin gelata to the Proto-Indo-European root *gel-, meaning cold or freeze, while fish comes from Old English fisc, ultimately from Proto-Indo-European *péysks.

The compound word appeared in the mid-19th century, replacing older names like sea jelly, blubber, sea nettle, and the scientific term medusa. Despite its name, a jellyfish isn't actually classified as a fish at all — it's simply an aquatic creature with a gelatinous body.

Jelly shares its frosty root with words tied to cold and freezing, since *gel- is the same PIE root behind gelū, the Latin word for frost.