(v): hit against; come into sudden contact with
(v): come upon, as if by accident; meet with
(v): dance erotically or dance with the pelvis thrust forward
(v): move in a pattern; usually to musical accompaniment; do or perform a dance
(v): assign to a lower position; reduce in rank
(v): give an assignment to (a person) to a post, or assign a task to (a person)
(v): remove or force from a position of dwelling previously occupied
(v): cause to move, usually with force or pressure
(n): a lump on the body caused by a blow
(n): any physical damage to the body caused by violence or accident or fracture etc.
(n): something that bulges out or is protuberant or projects from its surroundings
(n): any solid convex shape that juts out from something
(n): an impact (as from a collision)
(n): the striking of one body against another
In message boards and the like, bumping is the act of posting to push a particular thread to the top of the active list.
Some say it means "Bring Up My Post" but that's a backronym.
Probably an echoic for hitting