mustang

1. (US, military slang) A merchant marine who joined the U.S. Navy as a commissioned officer during the American Civil War.Examples: "He is a product of the merchant marine and is one of the officers called "Mustangs" who entered the navy during the Civil War."; "He was the son of a famous artist, and was what we termed a "mustang" officer, who had come into the navy from the merchant service during the Civil War."USslang

2. (US, military slang, generalized) A commissioned officer who started military service as an enlisted person.Examples: "[…] and the chief engineer is a "mustang" — that is, an officer who has risen from the ranks of enlisted men."; "Mustang— Officer who came up through the ranks. None better."USslang