1580s, "swindler, cheat," from Middle French imposteur (16c.), from Late Latin impostor "a deceiver," agent noun from impostus, contraction of impositus, past participle of imponere "place upon, impose upon, deceive," from assimilated form of in- "into, in, on, upon" (from PIE root *en "in") + ponere "to put place" (past participle positus; see position (n.)). Meaning "one who passes himself off as another" is from 1620s. Related: Impostrous. For a fem. form, Bacon uses French-based impostress (1610s) while Fuller, the church historian, uses Latinate impostrix (1650s).