The most extravagant reaction to something of funny, or toast-worthy nature, where someone uses their hand to make contact with their knee typically in a very absurd and flamboyant manner.
Guy: hey guess what
Guy#2: what?
Guy: UR MOM
Guy: performs a kneeslapper
A joke that's supposed to be funny but isn't really. And all you want to do is make the person not feel bad, but the joke isn't funny. So..... you just say "that's a real kneeslapper!"
something you use when a jokes not funny but you want to make the person feel better about themselves.
friend 1:why did the chicken cross the road
friend 2:idk why...
friend 1:to get to the other side
friend 2's mind:unfunniest shit ive ever heard god damn learn some humor
friend 2:wowee what a kneeslapper!oh geez you really got me with that one!!!