Billionaire

google

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mid 19th century: from billion, on the pattern of millionaire .

wiktionary

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From billion +‎ -aire.

etymonline

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billionaire (n.)

1844, American English, from billion on model of millionaire. Marked "[Rare.]" in Century Dictionary (1902). The first in the U.S. likely was John D. Rockefeller, some time after World War I.