Green’s Dictionary of Slang

(US) a large amount, usu. of money.

1949		W.R. Burnett Asphalt Jungle in Four Novels (1984) 177: Me with a sockful worrying because I’m getting set to clear twenty G’s.
1954		(con. 1940s) T.A.G. Hungerford Sowers of the Wind 188: We’ll get a sockful of sake.