Street Fighter in a book called Amongst The Marines

I recently read a book called Amongst The Marines by Steven Preece. Its an autobiography. This guy was sensational on the street with his nut, it appears to be one hell of a weapon. I thought his inception into his violent life style in the marines was horrendous, but the violent man he turned into must have been a result of that. Anyway, this guy was also a boxer and must have been lethal when caught in a tussle. Anybody else read this?

What in the heck is a nut (and why do I feel like I’m walking into the punchline of a dirty joke)?

A headbutt.