Definition
This idiom is used to express that a negative outcome has occurred as a result of previous actions. When someone says "payback's a bitch," it signifies that something bad has happened due to cause and effect.
報應是歸事（指做了壞事後會遭遇壞的報應）
He used to bully his classmates a lot, and now he's experiencing being bullied himself; payback's a bitch.
他以前常常霸凌同學，現在他自己也遭遇霸凌；報應真是個婊子。
She played many tricks to get promoted, and now she's being ostracized by her colleagues; payback's a bitch.
她以前用很多手段來獲得升職，現在卻被同事排擠；報應真是個婊子。
Their company didn't value product quality in the past, and now they're being boycotted by consumers; payback's a bitch.
他們公司以前不重視產品質量，現在被消費者抵制；報應真是個婊子。