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 perseverance and cunning, usually with out (adv.), is from 1570s. Related: Ferreted; ferreting.
ferret (n.)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 bear, carry." Also from the French word are Dutch fret, German Frett. Ferret-faced is from 1837 (to have ferret-eyes is from 1580s).
The word ferret was used in the English language in a 1398 translation by scribe John Trevisa because he couldn't find a word to stand in for Old French furet, with the same definition.
This term, which competed with ferretto be the proper spelling of the word for a while, comes from Vulgar Latin furittum, a diminutive of fur, meaning "thief".
The term was applied to the polecat in allusion to Roman perceptions of clever and sneaky attributes to the animal.