Knee Slapper

A saying a person does when another person says an unfunny joke but you're being sarcastic about it, indicating it was the funniest joke you have ever heard in your life. (being sarcastic if you didn't get that)

"Hey Billy, why did the chicken cross the road?... TO GET TO THE OTHER SIDE!" *laughs hysterically*

"What a knee slapper, Timmy!" *slaps knee*