early 15c., "hunt with a ferret," from ferret, n. or from Old French verb fureter, in reference to the use of half-tame ferrets to kill rats and flush rabbits from burrows. The extended sense of "search out, discover," especially by perseverence and cunning, usually with out (adv.), is from 1570s. Related: Ferreted; ferreting.

late 14c., from Old French furet "ferret," diminutive of fuiron "weasel, ferret," literally "thief" (in allusion to the animal's slyness and craftiness), probably from Late Latin furionem (related to furonem "cat," which also meant "robber"), from Latin fur (genitive furis) "thief," probably from PIE *bhor- (which likely also is the source of furtive), from root *bher- (1) "to carry" (see infer). Also from the French word are Dutch fret, German Frett. Ferret-faced is from 1837 (to have ferret-eyes is from 1580s).