Jounce, v. t. & i. [imp. & p. p. Jounced; p. pr. & vb. n. Jouncing.]
Etymology: [Cf. Jaunce.]
To jolt; to shake, especially by rough riding or by driving over obstructions.
1 other Webster entry under this spelling
Jounce, n.
A jolt; a shake; a hard trot.
Shades of meaning
Fernald has no entry of its own for jounce; he lists it under shake.
He lists it without comment; the entries above explain its neighbors.