To headbang is to throw your head up and down, hard, in time with loud rock or metal music — the kind with heavy drums and guitars. People do it at shows, and at home with the music up. It is informal. You use it as a verb: they headbanged through the whole song. Though actually, the older word is headbanging; English later cut that down to the shorter verb headbang. The tidy line in some books points at the person who loves that music and does this; that person is a headbanger, a different word. Friends may joke that you are headbanging if you nod along too wildly to a pop song, but a small happy nod is not this. It is not hitting your head on a wall, and it is not hitting another person with your forehead.

甩頭（聽搖滾、金屬樂時把頭猛力上下甩）