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"
I was laughing at this guy behind this one dude and the dude thought I was laughing at him so this nigga had the nerveto BUCK UP to me and I had to lay his ass down.