What is the origin of the idiom "fair-weather friend" and when to use it?

The idiom "fair-weather friend" is derived from the fact that a fair-weather day is a day when the weather is mild and pleasant. Similarly, a fair-weather friend is someone who is only around when things are going well. It is used to describe someone who appears to be a friend but abandons or turns against you when you face hardships or adversity. It highlights the insincerity or lack of true friendship.