What is "head-butting"

Usage examples of "head-butting".

Outside the picture houses, curry houses, and Chinese takeaways of Peckham of a Friday night, there was nothing but characters head-butting lampposts and each other to Bruce Lee sound effects.

He didn't have the stamina to put away two pitchers of Kingsland, play a decent game of pool, drink two more pitchers while talking about the influences of Sherwood An-derson and Gertrude Stein on the young Hemingway, get into some serious head-butting, put down another couple of pitchers, emerge clearheaded enough to go barrel-assing through the countryside, pick up a couple of experimental Madison girls, smoke a lot of high-grade shit, and romp until dawn.