Someone with whom you do not have a relationship with but you have mutual sexual relations . Usually using the abbreviation FB, although, FB is sometimes mistaken for Facebook or Finger bang .
Most of the time, both members of the sexual relation are single and their relation is strictly for only that purpose.
Ex 1 - Hey dude, I am going to hang out with my fuck buddy today
Ex 2 - Hey, I know this is sudden, but do you want to be my FB Fuck Buddy?
Someone you meet up with occassionally to get frisky. Generally, there are no special feelings attached to them (other than to their genitals). They fulfil your sexual needs...at least a little bit. Haha....
A: Dude, totally met up with the Minute Man last night!
You're not really buddies, you just fuck, usually only as a booty call after you failed to pick up during the 3 o'clock scab grab when last drinks were called at the pub. Dropping the word buddy and replacing it with person (you could use a gender specific noun if desired) is far more appropriate.