What is the opposite word of silly?

The opposite of silly depends on the specific context. Here are a few options:

General opposites:

* Serious

* Wise

* Sensible

* Intelligent

* Mature

More specific opposites:

* Funny (if silly is used to mean "unfunny")

* Smart (if silly is used to mean "stupid")

* Sophisticated (if silly is used to mean "childish")

* Appropriate (if silly is used to mean "inappropriate")

The best opposite will depend on the exact meaning you want to convey.