It is a somewhat old fashioned way of saying "just
kidding." It
sounds more Shakespearean than "
jk d00000d."
Person A: Bitch, I'll kick your ass!
Person B: *Gasp* You have such a sailor mouth!
Person A: What the motherf**k??
Person B: Har
har, I kid, you old sea
dog!
Person A: Har har har
har!