I am laughing my head off.
我都要笑掉大牙了。

Scene
Jane: How was the performance last night?
Carrie: Great! It was really funny. We were laughing our heads off.

Notes
呵呵，看到这句话时你是否捏了一把汗？笑会掉脑袋吗？^_^ 中文有笑掉大牙的表达，到了英语里头为什么却成了掉脑袋！ 因为英美人认为，人笑时，头在摇晃，前俯后仰地，头都快掉下来了！所以， “laugh one's head off" 的意思实际是" laugh loudly "， 可译为“笑掉了大牙”。

Exercises
现在来看看这些短语又是什么意思呢：
talk sb's head off
eat one's head off
cry one's head off
shout one's head off。