Severe form of mental illness. To call it a mood swing is a understatement. It more like PMS on crack. Bipolar highs are characterized by delusions of grandure(thinking you can do more than you are capbable of). Lows are extreme anger or sadness.
Believe me. I live with some with the illness.
I don't care how hot she is -- that bipolar bitch would cut you dick off in the middle of the night for leaving the milk out.