1570s, "weasel-like animal of Egypt," from Latin ichneumon, from Greek ikhneumon "ichneumon," literally "searcher, tracker," perhaps so called because it hunts crocodile eggs, from ikhneuein "hunt for, track," from ikhnos "a track, footstep, trace, clue," which is of unknown origin. Used by Aristotle for a species of wasp that hunts spiders (a sense attested in English from 1650s).

"snake-killing ichneumon of India," 1690s, perhaps via Portuguese, from an Indic language (such as Mahrathi mangus "mongoose"), probably ultimately from Dravidian (compare Telugu mangisu, Kanarese mungisi, Tamil mangus). The form of the English word has been altered by folk-etymology.