Look a gift horse in the mouth
指对别人给予的好处或恩惠吹毛求疵。
批评对恩惠挑剔
英文释义
To find fault with something that has been received as a favor.
例句
She complained about the free tickets, which is looking a gift horse in the mouth.
典故来源
源自马匹交易，看牙知龄，但礼物马不应挑剔，比喻不领情。