To challenge someone by getting in their face and disrespecting them with belligerent comments or trying to stare the them down.
I was laughing at this guy behind this one dude and the dude thought I was laughing at him so this nigga had the nerve to BUCK UP to me and I had to lay his ass down.
by Joshiro007 February 12, 2003
Buck up means to basically grin and bear it, kinda like sayin "get over it" only more nicely. It's especially used in the south and often in small towns.
Mark was complainin about the rain ruinin the huntin trip and his dad said "it'll be ok, just buck up"
by libbylee2487 April 23, 2005