underdo

Underdo means to do something insufficiently or inadequately—the opposite of overdo.

If you underdo a steak, you don't cook it long enough; if you underdo your preparation for an exam, you haven't studied enough.

The word appears most often in its past participle form: The roast was underdone and practically raw in the middle.

While overdo is common, underdo remains relatively rare, perhaps because English already has plenty of words for falling short.