ichneumon n.

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).