A mindless person or someone with no personality; gives blank stares, especially when presented with ideas or facts that should arouse some sort of reaction in a sentient being. Basically brain dead or mentally challenged. It would be improper to use this against someone who has a real diagnosed disability of course. More like, if you meet someone in a professional capacity whose title suggests they are intelligent, but really they just stand around looking dumb.
"I told Guido that the server was down and he said 'So...?'. Is that guy a fucking flatliner?"