jounce (v.) Look up jounce at Dictionary.com
"to jolt or shake," especially by rough riding, mid-15c., of unknown origin, perhaps a blend of jump and bounce. "Several words in -ounce, as bounce, flounce, pounce, trounce are of obscure history" [OED]. Related: Jounced; jouncing. The noun is 1787, from the verb.