Definitions of tumbler:

• noun:   a glass with a flat bottom but no handle or stem; originally had a round bottom
• noun:   a movable obstruction in a lock that must be adjusted to a given position (as by a key) before the bolt can be thrown
• noun:   a gymnast who performs rolls and somersaults and twists etc.
• noun:   pigeon that executes backward somersaults in flight or on the ground