I actually just looked this up and found out some interesting things.
Young MacGuffin speaks in a North East Doric dialect which obviously is hard to understand. Originally he was going to just speak gibberish but his voice actor, Kevin McKidd, suggested he try a few lines in Doric and Pixar loved it and kept it. Found a couple translations too, but only one of them applies to this gif set and that’s the third one after Merida gave her speech about letting them choose who they marry:
“It’s just not fair making us fight for the hand of a girl who doesn’t want anything to do with it. You know?