The story
From midshipman + fish, after their prey-attracting photophores, which are said to resemble the buttons of a naval uniform.
Etymology from Wiktionary, CC BY-SA 4.0.
midshipman fish
Today it means: Any species of the genus Porichthys of batrachoid toadfishes, typically nocturnal, distinguished by photophores and four lateral lines, and noted for a hum produced by males during the breeding season. Full definition →