What is the opposite of word brief?

The opposite of the word "brief" depends on the context in which you're using it. Here are a few possibilities:

If "brief" means "short in duration":

* Long

* Extended

* Prolonged

* Extensive

If "brief" means "short in length":

* Long

* Detailed

* Comprehensive

* Extensive

If "brief" means "concise":

* Verbose

* Wordy

* Rambling

* Prolix

If "brief" means "summary":

* Detailed

* Comprehensive

* Full

* Extensive

So, the best opposite for "brief" will depend on the specific situation.