bief

in Old French bied, biet, biez, from Gaulish bedo- (canal, ditch) (see bez in Breton), from bhedh- "to pierce, dig" (see fodio in Latin, fosse in French, bed ("bed, riverbed") in English). The /d/ transformed into /f/ because it was initially intervocalic (between two vowels) before ending up at the end of the word.