jokingly
In a playful, not serious, just-for-fun way.
Jokingly means doing or saying something in a playful, humorous way rather than seriously. When you tell your friend jokingly that their new haircut makes them look like a poodle, you're teasing them in a friendly way, not actually trying to insult them. The word signals that you don't mean what you're saying literally.
Sometimes it's obvious when someone speaks jokingly: they might use a silly voice, exaggerate wildly, or grin while they talk. But other times people misunderstand, which is why you might need to add “I'm just joking!” if your tone wasn't clear. When you say something in jest (another way to say jokingly), you expect the other person to understand you're playing around.
The word comes from joke, and it describes the spirit behind what you're saying or doing. You might jokingly suggest eating ice cream for breakfast, jokingly challenge your teacher to a dance contest, or jokingly pretend to be a superhero. The key is that you are having fun, not making a real proposal or criticism.