Slang is the language you speak with your friends and usually it is not the thing you learn from your teachers. Not every slang has an exact Chinese counterpart, some some do or close to a Chinese slang.
“I’m pissed / I’m pissed off” — is a common expression saying you are not happy or, indeed, angry. In Chinese, you can say “我很生气”, which literally means “I am angry”. However the slang “不爽 bù shuǎng” is the slang expression.
For example, “I get pissed off when he lies to me” could be “ 我很不爽他骗我 “.
