means A fast aircraft propelled by engines that expel exhaust at high speed, or more broadly a forceful stream of liquid or gas shooting from a narrow opening.

from From French 'jeter,' to throw or cast, which comes from Latin 'iactare,' to hurl — a frequentative of 'iacere,' to throw. So a 'jet' was originally just something thrown out forcefully, like a spurt of water; only in the 20th century did the word leap onto aircraft, named for the thrown stream of exhaust that shoves them forward. (The black gemstone 'jet' is unrelated — that comes via Latin from the town of Gagas in ancient Lycia.)