fell (v.) Look up fell at Dictionary.com
O.E. fællan, (Mercian) fyllan (W.Saxon) "make fall," also "demolish, kill," from P.Gmc. *fallijanan (cf. O.N. fella, Du. fellen, O.H.G. fellan), causative of *fallan (O.E. feallan, see fall (v.)), showing i-mutation. Related: Felled; feller; felling.
fell (adj.) Look up fell at Dictionary.com
late 13c., from O.Fr. fel "cruel, fierce," from M.L. fello "villain" (see felon). Phrase at one fell swoop is from "Macbeth."