Headbutt is a British term, how do you say the same thing in American English? I am interested in both the verb and the noun.
The meaning of the constituent words head and butt are obvious enough, and the concept of butting heads is quite commonplace, so no American native speaker would likely be confused by the compound term headbutt.
The headbutt seems to be more common in association football (soccer) than in the major North Americans sports. It isn't a named infraction in any sport I'm familiar with — it would just fall under intentional foul or boarding or unsportsmanlike conduct and so on, like a punch or a kick or other physical strike. I would thus be unsurprised to see a higher prevalence in British as opposed to American usage, and the reverse true, say, for beanball or backcourt.
A COHA search turns up no instances of headbutt — but it does return head-butt and head butt from the 1990s onwards. What may have occurred is that head butt being more common in the UK, the closed compound was lexicalized earlier than in the US, where headbutt in journalistic sources only took an upswing after Zinedine Zidane headbutted Marco Materazzi in the 2006 FIFA World Cup final.
While this is just a theory, a crude Ngram suggests that head butt remains more common than headbutt in works printed in the U.S. — but also that head-butt is still more prevalent than headbutt in the British corpus as well, as of 2008: