The story
From Classical Latin sextus (“sixth”) + -illion; originally the sixth power of a million, 10³⁶. Compare slightly earlier quintillion, Middle French sixlion (1484) and French sextillion (1765). First attested in 1689 in John Locke’s An Essay Concerning Human Understanding (see the quotation).
Etymology from Wiktionary, CC BY-SA 4.0.