Answer:
These two might seem like straightforward antonyms, but there are a few differences to keep in mind. Overwhelm is a verb that means “to overpower” or “to cover or bury.” Underwhelm means “to fail to impress.”
Explanation:
Whelm is labeled as "archaic" in NOAD, as it has fallen out of use. Left in its wake are the would-be superlative overwhelm (which, rather than actually meaning "more than whelmed", has simply taken over its parent's definition) and its opposite underwhelm.
