Green’s Dictionary of Slang

an uncountably rich person.

1944		D. Runyon Runyon à la Carte 23: A rich zillionaire from the city of New York.
1952		W. Winchell On Broadway 4 Aug. [synd. col.] The zillionaire entered into a separation agreement with his wife.
1967		(con. 1920s) J. Thompson South of Heaven (1994) 123: Give my regards to that zillionaire daddy-in-law of yours.