Someone that, when you're trying to do something you don't want other people seeing, always seems to walk in and/or be hanging arround, preventing you from doing said thing.
Such people are often buzzkill and don't realise it.
Man, everytime my girlfriend and I try to make out, we keep getting a lurker.
Sorry dude, I can't click on that page; I have a lurker.
A person who didn't get an invite to a keggar who hangs around outside the property line on his KMart skateboard hoping some Humboldt Nuggetswill fall out of someones pocket for a ground score.
Spacin Jason is such a Lurker you can't get rid of him and his Boy/Girlfriend 'Fast' Eddy Cocks with a rifle shot over the head.
someone (like f99molina) who quietly watches the soap opera of collegiate message forums unfolds to be kept up to date and on point with anyone and everyone.