In the ancient days, MOB stood for movable object block. Also known as a sprite. This basically meant anything in the video game that moved. Your ship, enemy ship, etc.
In the MUD days, that changed to NPCs, as they could move from location to location and interact with PCs. Wheras normal objects could generally only be acted upon.
Most of these terms are MMO-based, and they do have some commonality in p&p RPGs.
As somebody also noticed, there is a tendency for those who use slang to scoff at those who don't know it. This is generally a rude behavior, as knowing some made up words does not make one superior to somebody else.
