One of the mysteries of the English language finally explained.
Petroleum, crude oil.
Mid 17th century; earliest use found in Walter Charleton (1620–1707), physician and natural philosopher. From rock + oil, ultimately after post-classical Latin petroleum petroleum. Compare German Steinöl, early modern Danish, Danish stenolie, Swedish stenolja, all now historical, which likewise show calques on the Latin word.
rock oil/ˈrɒk ɔɪl/
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