A picture, drawn in physics classes, that shows each force acting on an object. The object is usually shown by a dot, and only shows the forces acting on THAT object, hence the name, "free-body" diagram.
Let's just draw a FBD on the board here of a ball rolling down a ramp
An fbd is a facebook douchebag. This includes males with too many friends at least 150 of which they will never converse to or a female with an excess of friends at least 200 of which they will never converse with
That guy thinks he is cool because he has so many "friends" on facebook but he is really a fbd.