A person who is easy, a slut, a whore.
That girl I was with last night was a total fuckpants.
He's such a fuckpants.
someone who is being a flat out meanie face cockmaster by either stealing something valuable, talking behind your back, etc... usually the person who needs to be cockslapped to teach them a lesson.
frank: that guy josh is a fuckpants
jonny: i know dude we should beat his ass
frank: thatll teach him to hit my girl