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Results found in several dictionaries: Collins, Collaborative Dictionary
fell [3]  adj 
with one fell blow  con un golpe feroz 
at one fell swoop  de un solo golpe 

Collaborative Dictionary     English-Spanish
fall foul nnpl. chocar Reverso General Dictionary 
falling market n. el mercado bajista Reverso General Dictionary 
niagara falls n. las cataratas del Niágara Reverso General Dictionary 
sharp fall n. la caída repentina Reverso General Dictionary 
fall foul of nnpl. chocar con Reverso General Dictionary 
fall in love vt. enamorarse Reverso General Dictionary 
fall into ruin vt. caerse en ruinas Reverso General Dictionary 
fall of night n. el anochecer Reverso General Dictionary 
fall stone dead vt. caerse muerto Reverso General Dictionary 
fall to pieces vt. caerse a pedazos Reverso General Dictionary 
fall into the habit nnpl. acostumbrarse Reverso General Dictionary 
fall on hard times vt. atravesar tiempos difíciles Reverso General Dictionary 
fall upon hard times vt. conocer tiempos difíciles Reverso General Dictionary 
fall with a whop vt. caer como un plomo Reverso General Dictionary 
to fall into ruin vt. caerse en ruinas Reverso General Dictionary 
as easy as falling off a horse exp. más fácil imposible Reverso General Dictionary 
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'fell {3}' also found in translations in Spanish-English dictionary
desbarrancar v. 1) fling over a precipice (vt), 2) fall over a precipice (refl) Reverso General Dictionary 
de tal palo tal astilla adj. an apple does not fall far from the tree Reverso General Dictionary 

"Collins Spanish Dictionary 8th edition published in 2005 © William Collins Sons & Co Ltd 1971, 1988 © HarperCollins Publishers 1992, 1993, 1996, 1997, 2000, 2003, 2005"