As adjectives the difference between clinching and unimpeachable
is that clinching is that settles something (such as an argument) definitely and conclusively while unimpeachable is not able to be impeached or reproached.
As a verb clinching
is present participle of clinch.
clinching
English
Verb
(head)
Adjective
(-)
That settles something (such as an argument) definitely and conclusively