I recently had occasion to look up the etymology of "overwhelmed". I figured that since one could be "overwhelmed" or "underwhelmed", there must be some normative state of being "whelmed". I discovered the OE link with helmet, i.e. "to be covered". OK, understandable.

But then I realized that the "under" in "underwelmed" was a logical variation not on the root word, but only on the prefix, and etymologically made little sense.

Is there a name for such words? I would define them as non-etymological arbitrary, sometimes even contradictory variations.