The story
From a humorous association of the term fossil fuel with dinosaurs. This is technically incorrect, since petroleum is derived primarily from fossilized zooplankton and algae.
Etymology from Wiktionary, CC BY-SA 4.0.
dinosaur juice
Today it means: Any liquid (gasoline, diesel, mineral oil etc.) derived from fossil fuels (especially petroleum), commonly used in vehicles as fuel or lubricant. Full definition →