1) An otherwise unfunny,
awkward, or just stupid joke. Usually a dad pun or something of the sort. Said after this to show that it should have never been said in the first place. Invented by hillbillies and rednecks when sitting on their porches, slapping their knees when somthing
funny happens.
2)said after something
funny and stupid happens.
1)
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!"
2)
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)
skating
guy:"yea!"(slaps knee very hard, and dislocates it from the earlier accident)
friend:"no that's a knee slapper!"