The highland kiss

A Scottish offensive measure where one individual smashes his/her forehead into the nose of another individual.

Person A - "Ye daft bastard I'll punch ye square in the face!"
Person B - "No need ta fight laddie. How's about I Ey just give you the highland kiss!"
At which point Person B would grab the head of their enemy and slam his or her forehead into the central face area of Person A. Effectively ruining their day.