BUMP can be an acronym for Bring Up My Post.
Many Internet message boards arrange their threads so that the discussions with the most recent activity appear on the top of the screen. As a result, lively threads with active discussion are easiest to find; to see discussions that are a little more sedate, one must scroll down or click a button to go to earlier screens.
On boards that are set up this way, a thread can quickly disappear from public view if other threads take off in popularity. Like the New Writeups nodelet here on everything2, how long your favourite thread stays visible is partially a matter of luck. If someone floods the board with new topics, it may cause a perfectly good post to get pushed past the bottom of the screen.
Some people get aggravated when one of their posts disappears more quickly than they expected. These people deal with their primitive aggression by writing a response to their own post that contains the single word BUMP. The software, noticing "activity" in the thread, "bumps" the post up to the top.
Of course, many of the posts that fall off the bottom of the activity list do so, not because of bad luck, but rather, because they are boring, irrelevant, or not worth reading for other reasons. Such a post is not likely to bring about genuine discussion by means of a BUMP. However, the Internet is full of people who do not recognize this fact, and who continue to write BUMP-BUMP-BUMP as footnotes to their own posts in a desperate attempt to get attention. As if that's not bad enough, occasionally a catalogue of BUMPs is interrupted by whiny pleas for responses.
"Anyone out there?"
"What do you think?"
"Hello?"
"BUMP"
"BUMP"
"Hello?"
"BUMP"
Oddly, some of these people find it surprising when a thread made up of seventeen posts -- thirteen of which are BUMPs and three of which are tantrums, all attached to a message that was pointless to begin with -- does not endear them to the board moderators.