c.1200, from O.Fr. fals, faus, from L. falsus "deceived, erroneous, mistaken," pp. of fallere "deceive, disappoint," of uncertain origin. Adopted into other Gmc. languages (cf. Ger. falsch, Dan. falsk), though Eng. is the only one in which the active sense of "deceitful" (a secondary sense in L.) has predominated. Falsies "padded brassiere" first recorded 1943.