A headrush is a sudden rush of feeling in your head — pleasure, excitement, or a spin — that arrives all at once. The tight meaning is the high that can hit soon after someone takes a drug such as ecstasy or cocaine. In loose talk, people also use it for any sudden head-spinning wave: standing up too fast, a sharp drop on a ride, or a burst of excitement with no drugs at all. It is informal slang. You would not put it in a school essay, and it is not a doctor's word for a headache. Head here is the body part; rush is the feeling that comes in a hurry.

rush、上頭的快感（口語；原多指毒品剛發作時衝上來的那一下）