What word means the opposite of fashionable?

There isn't one single perfect opposite for "fashionable." It depends on what aspect of fashion you're emphasizing:

If you mean "not trendy":

* Unfashionable

* Out of style

* Dated

* Old-fashioned

* Outmoded

* Unstylish

If you mean "not stylish":

* Unstylish

* Drab

* Plain

* Unrefined

* Tacky (negative connotation)

If you mean "not popular":

* Unpopular

* Out of favor

* Neglected

* Ignored

If you mean "not sophisticated":

* Simple

* Basic

* Unpretentious

* Casual

The best word will depend on the context and the specific nuance you want to convey.