Headbutt is an ability available to members of the Brotherhood of the Fist.
Your hands and fists aren't the only methods of attack. A true brawler uses every part of his body, even his head.
Brawlers who learn the headbutt ability will be able to use the headbutt command, as well as occasionally use a headbutt as part of brawling combo. Headbutts to vital parts of the opponents body may cause a slight stun.