It is one of the best bits of derivational morphology I know!

And the best part is that it is part of the explanation of where the word “disgruntled” comes from. Turns out there really is a word “gruntled”, or well, there was - it’s fallen out of usage. “Gruntle” meant “to emit many small grunts” - as a pig might do if it were content. So disgruntled came to mean the opposite of being content.