JK stands for just kidding, meaning I'm only joking. It goes after a line that shouldn't be taken seriously: a tease, a prank, a deliberately scary piece of news. People write it only in texts to friends; out loud, they say the full just kidding or I'm only kidding.
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statement + jk; jk, + what is really true; (spoken) just kidding / I'm only kidding
- I ate all the cake you left in the fridge. jk, it's still there.
- Can't come tonight, I have to wash my hair. jk, see you at eight!
- Don't worry, I didn't tell anyone about the surprise. Well, only your mum. jk!
What changed
First versionDon't be upset, I just kidding! Your haircut looks great.
RevisionDon't be upset, I'm just kidding! Your haircut looks great.
In the full phrase, am is required: I'm just kidding. It's easy to drop because the short version, Just kidding!, has no subject or verb at all. But I just kidding, with a subject and no am, isn't grammatical.
The same verb, kid, gives two common phrases with a different meaning. I'm not kidding means I'm serious: I'm not kidding, the shop closes at five today. You're kidding! sounds like a surprised no way! in reply to someone else's news. Jk isn't written in messages to a teacher or colleague, or in essays; in speech, including the exam, the full phrase I'm only joking works.
Your turn
I sold your bike this morning! — Are you serious?! — ___, it's still in your garage.
Show the answer
jk — You scared your friend with news about his bike, then admit yourself that the bike is still in his garage. So the first line was a joke, and jk is needed. Fr means seriously or for real: with it, you'd confirm the sale, which contradicts the second half of the reply.
Play a prank on a friend by text: give them some alarming news about something of theirs, then add jk and say what's really going on.
I spilled coffee all over your new laptop. jk, it's on your desk and totally dry.
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