Green’s Dictionary of Slang

a sanitarium, esp. at a boarding school.

1914		‘Ian Hay’ Lighter Side of School Life 71: Broken neck, inflammation of the lungs, ringworm, and leprosy [...] You are going to the San.
1937		P. Morris [trans.] Bernanos Diary of a Country Priest 299: [W]hen I came out of the san., I had to find a job.
1940		B. Appel People Talk (1972) 268: You’re not supposed to be admitted to a T.B. San unless you have T.B.
1954		J. Blake letter 12 Jan. in Joint (1972) 38: He was put on the ad-lib list at the San.
1962		C. Clausen I Love You Honey, But the Season’s Over 213: I’ve got to rest, get rid of this cough. May be moved to another place, a san ...
1989		T. Blacker Fixx 67: No action would be taken against those reporting to the San.
1990		S. Geason Shaved Fish 119: They must have some san for Catholic drunks somewhere.