Definitionsbackside, the posterior, the ass. See ass for synonyms.
2. Metaphorically speaking, the inner works and workings of a person that can be turned on or off or accelerated.
(1) Jerry (Jack Lemmon) watching Sugar Kane (Marilyn Monroe) walk in Some Like It Hot (1959): ' Look how she moves. Just like Jell-O on springs. She must have a built-in motor or something.'
(3) Lois Laurel (Marilyn Monroe), Barnaby Fulton (Cary Grant) in a car in Monkey Business (1952):
-- Lois: ' Is your motor running? '
-- Barnaby: ' Yes, is yours? (Races the engine of his car.) Takes a while to warm up.'
-- Lois: ' Does me too.'
(4) Rusty Martin (Ann-Margret) to Lucky Jackson (Elvis Presley) in Viva Las Vegas (1963):
-- Rusty: ' I'd like you to check my motor. It whistles.'
-- Lucky: ' I don't blame it.'
(5) Bugsy (Scott Baio) to Tallulah (Jodie Foster) in Bugsy Malone (1976): ' Careful! You're racing my motor! '