tum·bler 
(tŭmblər)
(tŭmblər)n.
1. One that tumbles, especially an acrobat or gymnast.
2.
a. A drinking glass, originally with a rounded bottom.
b. A flat-bottomed glass having no handle, foot, or stem.
c. The contents of such a drinking glass.
3. A toy made with a weighted rounded base so that it can rock over and then right itself.
4. One of a breed of domestic pigeon that characteristically tumbles or somersaults in flight.
5. A piece in a gunlock that forces the hammer forward by action of the mainspring.
6. The part in a lock that releases the bolt when moved by a key.
7.
a. The drum of a clothes dryer.
b. A tumbling box.
8.
a. A projecting piece on a revolving or rocking part in a mechanism that transmits motion to the part it engages.
b. The rocking frame that moves a gear into place in a transmission, as on a lathe.
[Sense 2a, from the fact that it would tumble if put down.]