Definition & Meaning of "[sleep] with the fishes"

What is the origin of the idiom "sleep with the fishes" and when to use it?

The idiom "sleeping with the fishes" has roots in mafia and gangster culture, particularly in the United States. It's often associated with organized crime and the practice of disposing of someone's body by dumping it into a body of water, typically to hide evidence of a murder. The exact origin of the phrase is uncertain, but it became popularized through its use in movies, television, and literature depicting underworld activities.