Disgruntled means annoyed and dissatisfied, usually because you feel you have been treated unfairly. Not furious — sour.
It attaches to customers, employees and former colleagues, and a disgruntled ex-employee is close to a fixed phrase in stories about leaks. The word implies grumbling rather than confrontation. There is a well-known joke about nobody ever being gruntled, and gruntle does exist as a rare back-formation, which makes the joke slightly less good and rather more interesting.
1adjectivedisgruntledAngry and dissatisfied, especially at unfair treatment.
“A disgruntled customer had left three reviews.”
From disgruntle + -ed.
Etymology from Wiktionary, CC BY-SA