Green’s Dictionary of Slang

1. one who behaves in a socially acceptable manner, usu. in a negative sense, i.e. lacking in anything other than polish; also as adj.

1902		H.L. Wilson Spenders xxxi 367: That’s right, son. I knew I could make something more than a polite sosh out of you [DA].
1949–51		in M. Daly Profile of Youth 108: He was known as ‘real sosh,’ short for ‘social’.
1972		B. Rodgers Queens’ Vernacular 86: Social butterfly with little individuality [...] sosh (fr social).
1989		P. Eckert Jocks and Burnouts 2: Jocks may have been ‘Soc’s’ (short for ‘Socialites’ and pronounced soshes).

2. (US teen) a social climber.

1967–8		Baker et al. CUSS 201: Sosh A socially adept person.
1984		J. Ellroy Blood on the Moon 223: ‘He wasn't a sosh and he wasn't an athlete and he wasn't a bad ass’.
1989		P. Munro Sl. U. 176: soc person with a superficial personality who is known for participating in a lot of social activities (usually a girl) [...] (plural soshes).