Green’s Dictionary of Slang

using the top of one’s head to butt an opponent during a fight; such a blow can often end the fight instantly.

1989		M. Amis London Fields 24: Only one real fight so far. An incredible flurry of fists and nuttings.
1999		N. Cohn Yes We have No 209: One [...] gets the other in a headlock and pantomimes a nutting.