The Collaborative International Dictionary of English v.0.48:

Jounce \Jounce\ (jouns), v. t. & i. [imp. & p. p. Jounced
   (jounst); p. pr. & vb. n. Jouncing (joun"s[i^]ng).] [Cf.
   Jaunce.]
   To jolt; to shake, especially by rough riding or by driving
   over obstructions.
   [1913 Webster]