When you are sat on the sideline of a tennis match and a good rally gets going. Then you head begins to bounce from side to side keeping up with the play until you a) give up and rest or b) are finally relieved by the players.
"Morning Jane, I have woken up with an awful neck cramp."
"Well what do you expect John. You were watching Murray yesterday. You have tennis head!"
or
"I love watcching Roger Federer , he always gives me great Tennis Head!"
It is one of the most athletic sports. It's not likebasketball where you just sit on the sidelines. You are constantly moving during matches and it takes skill. Matches can last up to 1-2 hours!
EX:
Stranger: I play basketball Me: I play tennis
Stranger: Tennis is stupid and its for old people Me: Punches them and runs
Nobody: ...