The part of one's brain that actually does the thinking-as opposed to the areas that deal with sex ("Me want FUCK!"), food ("Me want peanut butter cups."), relationships ("Me want FUCK!"), seeking shelter/safety (frequently negotiated via demand for/willingness to provide sex and peanut butter cups), fashion sense ("You're going to wear THAT?").
Taking the college entrance exam really strained my "thinkulator"-I can tell because I notice a "fizzing" sensation in my head, then my eyes and ears start crossing and uncrossing.
A mental/neurological disorder. The primary symptom: submitting one's own name (or that of an acquaintance) to the Urban Dictionary, usually defined as (for males): "great guy", "spunk pump with huge cock", "mini-micro dick", "an asshole supreme", etc. (for females): "fine lady", "slut", whore", "slut-whore", "skeeving skeezer", "cock tease", "bitch", etc. In the most serious form of the disorder, the patient actually thinks he/she/it is worthy of something beyond the withering contempt and ridicule heaped upon them by the editors.
This guy obviously suffers from "Definition Deficiency Syndrome"-he submitted his girlfriend's name last week and praised her, but now she's a cankerous, syphilitic slut with buck teeth and excess nose hair.