The unpaired word.

Many English words are constructed from various parts like roots, prefixes and suffixes.

For example consider overwhelmed, underwhelmed, reckless, debunk, disgusted, innocent, dismantle, related, unscathed and nonsensical.

These words are called unpaired because the original root word rarely if ever appears alone in common usage.  An example is the base word whelm which comes from the Ole English hwielfan meaning to completely submerge in water.  The original meaning of whelm was made stronger by adding ‘over’ or ‘under’ in front of it.  The use of whelm faded away never to be unfaded.  (heh)

I was not aware that such constructions have actual names, but there you have it.

And then to drive you completely mad consider that ravel and unravel mean the same thing, as do fat chance and slim chance.

Attempting to make English sensical will leave you scathed.