A mistaken belief, especially based on an unsound argument
Base and Suffix of the Word
Fall- The root of fall means trick.
Acy- A suffix of nouns of quality, state, office, etc.
How Fall and -Acy Have Become a New Noun
The root fall, meaning trick, was joined with -acy which led to the definition of the suffix and root being slightly modified to the definition of fallacy, a mistaken belief.