Another way of saying Just Kidding, or J/K on IM related messages.
Cool Guy: Hey your fat!
Nice Girl: What! Never talk to me again!
Cool Guy: Jay kay!
Nice Girl: Haha, okay!
The pronounciation of the acronym jk, which stands for just kidding. Used as internet lingo, or, for really cool people, in real life conversations.
John: ...um...I think I might be gay.
Rob: Will you marry me?
John: jay kay! ...and...uh...no?