What is the origin of the idiom "be in each other's pockets" and when to use it?

The origin of the idiom "be in each other's pockets" is unclear. It likely emerged from the notion of two people being so physically close that they are constantly near each other, like items kept in the pockets of clothing. It is used to describe a situation where two or more people are constantly in close proximity, often to the point of being overly involved or dependent on each other. It implies a lack of personal space or independence between the individuals.