I love the word disgruntled, largely because of how it sounds and also because of its tricky prefix.
While I tend to think of dis- as only meaning “the opposite of”, according to Merriam-Webster’s fourth entry, dis- also means “completely”.
Gruntle, according to Merriam, means “grumble”, derived from “grunt”.
Disgruntled then, means to grumble completely, as opposed to stopping grumbling as one might think according to the far more common meaning of dis-.
How marvelous that we have a word describing a state of complete grumbling!