adverb. A term used to describe a dumb-ass joke. Its inception was the result of someone noticing their friend slapping their thigh/knee when a joke was told. It's an official "knee slapper" when the person laughing sqints their eyes, slaps their knee, and can barely speak due the amount of laughimg taking place. This definition is considered a "knee slapper".
What do you call someone who doesn't fart in public?
A private tutor!!