laugh up one's sleeve
形容某人表面同情实则暗喜
例句
While pretending to sympathize, she was laughing up her sleeve at his misfortune.
典故来源
源自16世纪，当时人们用长袖遮住嘴巴偷笑，避免被察觉，后引申为暗中幸灾乐祸。