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!
B: What?! I thought you said the sex was crap?
A: Meh...I needed to get laid. He's my fuck buddy until I find something else hottie hunting.
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.
I feel like a root but can't be fagged trying to find one. I'll call up my fuck buddy.