don't look a gift horse in the mouth 字面意思是“赠马不看牙”，它建议人们对别人的礼物不要不屑一顾，不要嗤之以鼻或不要忘恩负义。

【例句】
Don't look a gift horse in the mouth, be grateful for what you have received.
不要忘恩负义，要对你收到的东西心存感激。

I know you don't like the dress very much, but it was a gift; you should not look a gift horse in the mouth.
我知道你不太喜欢这件衣服，但那是一件礼物，你不应该对礼物嗤之以鼻。