laugh up one's sleeves
偷偷地笑
A:What's wrong with your face?
A:你的脸怎么了?
B:My brother did it.
B:我哥打的。
A:Why?
A：他为什么打你?
B:I laughed up my sleeves at him. He's seen his girlfriend off at the airport and had lipstick on his face and didn't know it.
B：我偷偷笑话他来着，因为他刚去机场送他女朋友，脸上留下了口红印，他还不知道。