Определение и Значение "[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.