What is the opposite of word humorous?

The opposite of "humorous" depends on what aspect of humor you're focusing on. Here are a few possibilities:

If you're thinking of "funny" or "amusing":

* Serious

* Solemn

* Dour

* Grim

* Somber

If you're thinking of "lighthearted" or "playful":

* Serious

* Earnest

* Formal

* Strenuous

* Grave

If you're thinking of "witty" or "clever":

* Dull

* Boring

* Uninspired

* Tedious

* Flat

If you're thinking of "ironic" or "satirical":

* Straightforward

* Literal

* Direct

* Serious

Ultimately, the best opposite for "humorous" depends on the specific context.