不去了。

You can use it as a standalone sentence. If a friend asks if you're coming to the party, just say 不去了. You can also add a reason before or after it. For example, 太累了，不去了 (Too tired, not going). It works perfectly in person or over text. It is short and punchy, just like English speakers saying "I'm out."

Use it when plans change last minute. Use it when you are too tired to leave the house. It is great for declining a second round of drinks. It also works when a destination looks too crowded or boring. It is the ultimate "opt-out" button for social obligations. Use it with people you know well for the best results.

Avoid using this with your boss for a mandatory meeting. It sounds a bit too casual and dismissive. Don't use it for very formal invitations without a long apology. If you use it with a romantic partner without an explanation, you might start an argument! It can sound a bit blunt if you don't soften it with a "sorry."

Chinese culture values "face," but it also values efficiency among close friends. 不去了 is part of the modern, fast-paced urban life. People are busy and tired. This phrase reflects a shift toward being more direct about one's energy levels. It is the verbal equivalent of the "JOMO" (Joy of Missing Out) movement in China.

You might hear 我不去了 to emphasize that *you* specifically aren't going. If you want to be polite, add 不去了，下次吧 (Not going, maybe next time). If you are feeling extra lazy, you might just say 算了，不去了 (Forget it, I'm not going). These variations help you tweak the mood from "busy" to "totally over it."