So-so (adjective)

1. Neither good nor bad; mediocre.
2. Neither one thing nor the other; indifferent.

Origin:

Of uncertain origin, first recorded in the early 18th century.

Examples:

1. The movie was so-so, not terrible but not great either.
2. How was the party? Oh, it was so-so.
3. The food was so-so, nothing special.
4. The performance was so-so, it lacked energy and enthusiasm.
5. The book was so-so, it didn't capture my attention.