From Old Japanese. Derivation uncertain. Appears in the Kojiki in ambiguous reference to a kind of sea creature (possibly sharks or crocodiles). The sense of crocodile clearly appears in the Wamyō Ruijushō dictionary (938) with an explanation of “having four legs like a turtle and a three-foot mouth with sharp teeth.”
Various theories exist regarding the term's origin, such as an alteration of 鬼(oni, “demon, devil”), or an abbreviation of 海主(watanushi, “sea master, sea god”).