What is the opposite of half-witted?

The opposite of half-witted depends on what aspect you're focusing on:

If "half-witted" means "stupid" or "lacking intelligence":

* Intelligent

* Smart

* Brilliant

* Astute

* Sharp

* Clever

* Wise

If "half-witted" means "naive" or "lacking common sense":

* Sophisticated

* Worldly

* Shrewd

* Savvy

* Discerning

* Astute

If "half-witted" means "lacking focus" or "inattentive":

* Attentive

* Focused

* Sharp

* Alert

* Observant

Ultimately, the best opposite of "half-witted" will depend on the specific context in which you are using the word.