One's buttocks. A "caboose" is the last car on a freight train. Get your caboose in here, dinner's ready!Oh please, there's no way my big caboose will fit into pants that size.
caboose
(kəˈbus)
n. the buttocks. (From the name of the car at the end of a railroad train.) You just plunk your caboose over there on the settee and listen up to what I have to tell you.