Someone (usually an emo) who claims to have bipolar/manic depressive disorder, in where it's really not true at all.
"Dude, did you see that chick with pink highlits and shit?"
"Yeah, must be a goth--I heard she's bipolar or something."
"Nah, man...she's biposer...she was just blowing smoke out her ass when she told you she's manic depressive."
"That's fuckin' gay, man."