Causing sexual attraction or being sexually appealing to both sexes either as a result of specific physical features which resemble those of the opposite sex of the individual in question (see Brad Pitt's or Matt Damon's curvaceous, feminine butts) and/or through a persona that causes sexual attraction to both sexes (see Brad Pitt's performance in "Fight Club").
Noun: a person, normally a pubescent female, who is thoroughly convinced that said person is attracted to both genders, though they are really just confused. A bismexual person will probably admit to being straight within a few months of their original declaration.
Adjective: A way to describe someone who is pretending or believes (but isn't really) that said person is Bisexual
Person 1: Didn't Amelia say she way bi? Why is she only flirting with guys, even though there are hot girls over there?
Person 2: Oh, she got over herself last week,y'know, she was bisMexual. Now she's straight as an arrow.