2)said after something funny and stupid happens.
dumb guy:"Two peanuts are walking down the street, and one of them was assaulted!"
normal guy (slaps knee and fake-laughs):"Wow! That sure was a knee slapper!"
guy skates and falls off rail, hurting his knee and friend laughs.
Later they are talking and laughing about a dumb joke.
friend:"wow that sure was a knee slapper!" (slaps knee)
guy:"yea!"(slaps knee very hard, and dislocates it from the earlier accident)
friend:"no that's a knee slapper!"
Audience member in sarcastic tone: wow that's a real knee-slapper
A private tutor!!