Try Our Apps


The Best Internet Slang


[ef-i-key-shuh s] /ˌɛf ɪˈkeɪ ʃəs/
capable of having the desired result or effect; effective as a means, measure, remedy, etc.:
The medicine is efficacious in stopping a cough.
Origin of efficacious
1520-30; < Latin efficāci- (stem of efficāx) effectual. See efficient, -acious
Related forms
efficaciously, adverb
efficaciousness, noun
nonefficacious, adjective
nonefficaciously, adverb
unefficacious, adjective
unefficaciously, adverb Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2017.
Cite This Source
Examples from the Web for efficacious
Contemporary Examples
Historical Examples
British Dictionary definitions for efficacious


capable of or successful in producing an intended result; effective as a means, remedy, etc
Derived Forms
efficaciously, adverb
efficaciousness, noun
Word Origin
C16: from Latin efficāx powerful, efficient, from efficere to achieve; see effect
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Cite This Source
Word Origin and History for efficacious

"sure to have the desired effect" (often of medicines), 1520s, from Latin efficaci-, stem of efficax (see efficacy) + -ous. Related: Efficaciously; efficaciousness.

Online Etymology Dictionary, © 2010 Douglas Harper
Cite This Source

Word of the Day

Difficulty index for efficacious

Some English speakers likely know this word

Word Value for efficacious

Scrabble Words With Friends

Nearby words for efficacious