Definition & Meaning of "look out for number one"

What is the origin of the idiom "look out for number one" and when to use it?

The idiom "look out for number one" advises a person to prioritize their own interests and well-being above all else. It originated in the mid-20th century and is often used in a somewhat self-centered or survivalist context. This expression is used to emphasize the importance of self-preservation and the idea that one should take care of their own needs and concerns first, especially in competitive or challenging situations.