Somebody with a bad attitude.
Generally this term is used when the person is being rude towards you and you don't want to piss them off more so by calling them a bitter berry they cant get mad at you because its not an insult... its just funny.
Typical results with turn there frown up-side-down. They may even get the giggles.
You need to ripen up! You're being a bitter berry!
Prices shown in USD.