early 15c., from ferret (n.), in reference to the use of half-tame ferrets to kill rats and flush rabbits from burrows; the extended sense of "search out, discover" is 1570s. Related: Ferreted; ferreting.
late 14c., from Old French furet, diminutive of fuiron "weasel, ferret," literally "thief," probably from Late Latin furionem (related to furonem "cat," also "robber"), from Latin fur (genitive furis) "thief."