"Pissed off" is an informal and colloquial expression used to describe someone who is extremely angry, irritated, or upset about something. It is a stronger way of expressing anger than just saying "angry" or "upset." The term "pissed" itself is slang for being angry, and when combined with "off," it intensifies the emotion.
For example, if someone is treated unfairly at work and they are "pissed off," it means they are very angry and frustrated with the situation. It's important to note that the term is considered quite strong and may not be appropriate in all contexts or in formal settings.