commenting on a post or thread with the intent of stopping all future comments from the author or other participants. The thread bust usually goes off-topic of the original sentiment or twists it into a different meaning before ending all future coments.
post: the kids are at grandma and grandpa's for the week. time for some alone time with the wife.
commenter 1: that's awesome. you guys are "lucky ducks!"
commenter 2: I think you meant to say rabbits, not ducks.
this is a simple example and may not result in thread bust but shows the intent of thread busting.