Green’s Dictionary of Slang

(US prison/Und.) Elmira Prison, New York.

1951		D. Dressler Parole Chief 122: In de El (Elmira Reformatory) they used ta let me box in de gym.
1975		(ref. to 1940s) E. Torres Carlito’s Way 18: The end of the forties saw me into the slams at Elmira Reception Center, Elmira. Thereafter known as ‘the El’.