Green’s Dictionary of Slang

a major success, a show business ‘hit’.

1930		J.P. McEvoy Hollywood Girl 85: It’s just a long talking movie that’s all with a lot of sound and effects and love is just a big gag socko.
1939		W. Winchell On Broadway 8 Jan. [synd. col.] Joseph Mitchell’s [piece] frinstance, a socko.
1949		Wodehouse Mating Season 199: The whole aim of my foreign policy had been to the making of a socko by Esmond.
1951		Green & Laurie Show Biz from Vaude to Video 546: A boxoffice socko.
1973		Wodehouse Bachelors Anonymous 23: Triumph or disaster, socko or flop, he went on forever like one of those permanent officials at the Foreign Office.