指的是那些只在顺利时寻求友谊，但在困难或逆境中却不忠诚或不支持的人，可以理解为我们中文中常说的“酒肉朋友”、“不能共患难的朋友”。

Cambridge Dictionary中对fair-weather friend是这样解释的：
someone who is a good friend when it is easy to be one and who stops being one when you are having problems

例句
He has realized that Bill is a fair-weather friend.
他已意识到比尔是个不能共患难的朋友。