A fein is someone you're getting closer to, it's not official that you and this other person are in a relationship but you're still loyal to eachother.
Jason: oh bro I got a new fein last week.
Bradley: fuck, who?
Jason: that hot Jessica chick!!
Bradley: that's so good man.
a Fein is someone who always wants the chooch. This especially applies to someone who doesn't have one of their own, so they ask everyone else for 'just one rip'.