illustration of a Head Butt on page 133 of Ring Dream, which appears to be a Grab and Smash too.
Head Butt (aka headbutt) is a technique described on page 74 of GURPS Martial Arts.
Prior to it being defined there, there was passing mentions on B222 / B557.
Hurting Yourself applies to the Skull, but parries with weapons apply damage to the Face.