What is the opposite of not a joke?

The opposite of "not a joke" depends on what you mean by "not a joke."

Here are a few possibilities:

* A joke: This is the most straightforward opposite. If something is "not a joke," it's serious. Therefore, the opposite would be something that is intended to be humorous, like a joke.

* Serious: This is a more general opposite, meaning the opposite of lighthearted or playful.

* Funny: This emphasizes the humorous aspect of a joke.

So, the best answer depends on the specific context. What do you mean by "not a joke"?