What is the origin of the idiom "yellow streak" and when to use it?

The phrase "yellow streak" has its origins in the early 20th century and is derived from the association of the color yellow with cowardice or fear. It is used to describe someone who is believed to be lacking courage or exhibiting cowardly behavior. It implies that the person has a figurative "yellow streak" running through them, symbolizing their supposed fear or timidity.