From Citizendium - Reading time: 1 min
The term enlisted is applied to the lowest rank individuals in military services.[1]
The higher status of the ranks referred to as Commissioned officers and, warrant officers reflects the historical practice that these individuals had an actual document, a commission, or warrant, that specifically named them, and laid out their special duties and responsibilities.[1]
Enlisted personnel joined up by going to a recruiting depot, or recruiting officer, and, literally, putting their name on a list -- hence "enlisted".[1]