The term "buck" in football

[spartan4life]

I was wondering what exactly the term "buck" means in the game of football.I have looked it up and have found different answers:

1.When a running back takes a step to the side, in order to get out of the way of another running back, before getting the handoff

2.When a fullback charges straight into the line (more old school I believe)

I've seen and heard the term alot.But I am sure there is an actual definition for it.

If you have one I'd like to know thanks.

[therealtedseay]

Football resources going all the way back to the turn of the 20th century refer to a straight-ahead run by a back as a "line buck"...possibly because a bucking horse must get his head down?