What is an antonym for the word fun?

There isn't one perfect antonym for "fun" as it depends on the context. Here are some options depending on what you want to convey:

For enjoyment:

* Dull: Describes something lacking excitement or interest.

* Boring: Similar to dull, emphasizes a lack of stimulation.

* Tedious: Something that is long and tiring, making it unpleasant.

* Monotonous: Characterized by a lack of variety and excitement.

* Unpleasant: Describes something that is disagreeable or distasteful.

For lightheartedness:

* Serious: Something that is important, grave, or somber.

* Solemn: Expressing or characterized by deep seriousness.

* Grim: Describes something dark, forbidding, and depressing.

For playfulness:

* Work: An activity done for a purpose, often with a feeling of obligation.

* Duty: Something that is required or expected, often without pleasure.

* Responsibility: A task or obligation that requires attention and care.

The best antonym for "fun" will depend on the specific situation and the nuance you want to express.