In Microsoft Word, uncapable is marked as wrong. It sounded pretty alright to me, thus, I checked it up on the Web and found that many dictionaries do not have “uncapable” in their entries, but dictionaries such as Dictionary.com and Merriam-Webster list it to be a synonym of incapable. My question is: is uncapable considered a “proper” word?
If a dictionary has it, that dictionary is just trying to be so comprehensive as to include any word ever. However, incapable is the proper and original form, and furthermore, everyone uses it. I have never seen uncapable in use.
The rule of thumb to go by is: If you’re choosing between variants of a word, pick the variant most widely used and understood. So when you have to choose, choose incapable.