The idiom sleep with the fishes has been popular since the 1970s, though it has been in use longer than that. To sleep with the fishes means to be murdered, to be killed and the body hidden from the public. The expression sleep with the fishes is a popular term used in media that portray the American Mafia or organized crime. The image is of a murdered person’s body being thrown into a body of water in order to escape detection. The idiom sleep with the fishes was popularized in the 1970s in Mario Puzo’s novel and movie, The Godfather: “It’s a Sicilian message. It means Luca Brasi sleeps with the fishes.” The expression sleep with the fishes may go back much farther. Edmund Spencer used the phrase in the 1830s. There is even a passage in Homer’s The Iliad: “Make your bed with the fishes now…” Related phrases are sleeps with the fishes, slept with the fishes, sleeping with the fishes.
Among her old pals who will not be attending are Jerry Angiulo, Whitey Bulger and Nicky Giso, all of whom sleep with the fishes, as well as Stevie “the Rifleman” Flemmi, who is locked up for life in parts unknown. (The Boston Herald)
Francis Ford Coppola’s groundbreaking mafia classic—as well as its great sequel and the mediocre third film in the trilogy—will sleep with the fishes on January 1. (Esquire Magazine)
Not like a “sleep with the fishes” kind of Godfather, but more in the fairy vein — although he didn’t seem to be magical so much as whimsical. (The Peninsula Clarion)