Fell means fierce and cruel, in old use. A fell purpose, at one fell swoop, fell deeds.
It survives almost entirely in one fell swoop, which is Macbeth's, and where fell means savage rather than sudden. It is a different word from the verb to fell a tree and from a fell in the north country.
1adjectivefellFierce and cruel, in old use.
“The knight rode out to face the fell beast that had burned the village.”
fell is a quality.
From Old French fel, cruel or treacherous, the same root as felon.
Proto-Germanic
*fallijaną — to fell, to cause to fall
Proto-West Germanic
*fallijan
Old English
fellan
Middle English
fellen
English, today
fell
Built from
From Middle English fellen, from Old English fellan, fiellan (“to cause to fall, strike down, fell, cut down, throw down, defeat, destroy, kill, tumble, cause to stumble”), from Proto-West Germanic fallijan, from Proto-Germanic fallijaną (“to fell, to cause to fall”), causative of Proto-Germanic fallaną (“to fall”), from Proto-Indo-European h₃elh₁-. Cognate with Dutch vellen (“to fell, cut down”), German fällen (“to fell”), Danish fælde (“to fell”), Norwegian felle (“to fell”).
Etymology from Wiktionary, CC BY-SA