Green’s Dictionary of Slang

(US) the same thing as usual.

1926		Maines & Grant Wise-crack Dict. 14/2: S.O.S. – Same old story.
1928		H. Brackbill ‘Midshipman Jargon’ in AS III:6 454: S.O.S.—Same old slum.
1939		Honolulu Star-Bulletin (HI) 25 Mar. 33/1: Slide the SOS (same old stuff — usually gravy or tuna-fish on toast).
1944		P. Kendall Dict. Service Sl. n.p.: S.O.S. . . . same old stew.
1947		M.H. Boulware Jive and Sl. n.p.: S.O.S. ... Same old stuff.
1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 182: School dinners are [...] ‘S.O.S.’ (Same Old Slush) and ‘Y.M.C.A.’ (Yesterday’s Muck Cooked Again).
1984		B. Ellem Doing Time 197: S.O.S.: same old shit (for lunch and dinner).
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 S.O.S. Same old shit, eg for a prison meal.