Someone who joins one or more people in a public eating area, who is not invited or welcome, and immediately wrecks whatever conversation, vibe or mood that was occuring, often with their mere presence. A mealjacker normally has no idea he is jacking someone's meal; he normally believes that the person or group actually wants him to be there. This common misconception is often predicated on polite small talk and the social norm of not being able to actually tell someone to sit somewhere else. Closely related to the cock blocker, the meal jacker strikes only during meals, when his victims are most vulnerable and least able to flee. But her powers of social extinguishment are not limited to potential romantic interaction; the mealjacker virtually shuts down all social conversation.
You're at the dining hall, eating the slop and generally having a good time with a couple friends, when you spot "Jeffery" or "Margaret" coming toward you carrying a tray. Your stomach drops. You let loose with an inner wail and scream "Nooooo" inside your head. But much like when you see the cop on the side of the road, it is too late to do anything about it now. You brace yourself, whisper-shout to your friend "Mealjacker!" and grit your teeth, because you are about to be mealjacked.
Mealjackings often occur in college dining halls and high school cafeterias, but can occur in any open eatery with general seating. The jackees often feel violated and helpless, especially in a booth setting, with little or no means of escape. See also close talker
and cock blocker