late 15c., "deception, false statement," from Latin fallacia "deception, deceit, trick, artifice," noun of quality from fallax (genitive fallacis) "deceptive," from fallere "deceive" (see fail, v.). Specific sense in logic, "false syllogism, invalid argumentation," dates from 1550s. An earlier form was fallace (c.1300), from Old French fallace.