A message board user who, though they do not actually hold the position of moderator, has some twisted sense of duty which they feel they must fulfill. This user seems to believe that the moderator is a retarded monkey that needs to be led around by the hand and told which threads need to be locked or deleted.
The backseat moderator can often be seen jumping up and down and frantically waving their arms, trying to draw attention to a thread which surely is offensive and stupid to the highest degree and must be locked immediately. In reality, the real moderator has probably looked at the topic and seen nothing which crosses the line.
The backseat moderator may also be a backseat administrator, letting everyone know which users need to be banned. Though the backseat moderator is irritating, there is nothing which can be done unless the real authority on the board takes matters into its own hands, and ejects them from the board in a hilarious example of irony.
"This topic is dumb. Lock it please!"
"Ban this guy, he's a moron!"