Green’s Dictionary of Slang

(US Und.) solitary confinement.

1928		R.J. Tasker Grimhaven 57: So now Jock was in sol.
1930		Amer. Mercury Dec. 456: He draw sol till he’s [crazy] [W&F].
1949		Monteleone Criminal Sl. (rev. edn) 217: sol To be placed in solitary confinement.
1971		(con. 1920s) J. Brown Monkey Off My Back (1972) 9: On my inaugural night in ‘Old Sol’ I was in such bad physical condition.
1992		Bentley & Corbett Prison Sl. 12: Sol An abbreviation for solitary confinement.