is an extension of it's true meaning. someone who polices the halls and rats on fellow peers for petty power, self esteem and approval from elders. someone who gets off or generally thinks they are in charge by randomly busting you. self appointed judges of anything.
who do you think you are, the fucking hall monitor?
The lovable guy in the office whose lack of diligence is overlooked because he's so darn nice
When it comes to anything outside the realm of television, sports, or pop culture, Hall Monitors are boderline retarded, but they're so agreeable and good-natured that they appeal to everyone nonetheless.