An Etymological Dictionary of Astronomy and Astrophysics

1. An act of tumbling or falling.
   

   2a) To fall helplessly down, end over end, as by losing one’s footing, support, or equilibrium; plunge headlong.
   

   2b) To roll end over end, as in falling (Dictionary.com).

Etymology (EN): M.E. tum(b)len “to dance in acrobatic style;” perhaps from O.E. tumbian “dance about, tumble, leap;” maybe related to
Du. tuimelen, O.H.G. tumon “to reel.”

Etymology (PE): Kaltâv, from Kermâni keletow, Malâyeri kallatow “wobbling,” from kal, kalleh “head” + tâv, tow, tâb “swing, twist,” from tâbidan “to twist, to spin.”