Used by someone as a way to soften a direct insult by saying it was just a joke, although the person doing the insulting still means the insult after the Just kidding! has been said.
Reggie: "Bob, you're such an asshole!"
Reggie: "Just kidding!"
Bob: "Oh, ok."
Bob now doesn't feel he's been insulted, because he was told that it was a joke, but Reggie still means what he said.
A phrase used by preps when they feel as though they said something stupid. Used frequently. Extremely frequently.
"What's 2 + 2?" "It's 4 you dumbass." "Oh, just kidding!"