Why is the ninth month called September?
early 15c., "to prance, strut, swagger," from Middle French jeter "to throw, thrust," from Late Latin iectare, abstracted from deiectare, proiectare, etc., in place of Latin iactare "toss about," frequentative of iacere "to throw, cast," from PIE root *ye- "to do" (cf. Greek iemi, ienai "to send, throw;" Hittite ijami "I make"). Meaning "to sprout or spurt forth" is from 1690s. Related: Jetted; jetting.
"stream of water," 1690s, from French jet, from jeter (see jet (v.)). Sense of "spout or nozzle for emitting water, gas, fuel, etc." is from 1825. Hence jet propulsion (1867) and the noun meaning "airplane driven by jet propulsion" (1944, from jet engine, 1943). The first one to be in service was the German Messerschmitt Me 262. Jet stream is from 1947. Jet set first attested 1951, slightly before jet commuter plane flights began. Jet age is attested from 1952.
"deep black lignite," mid-14c., from Anglo-French geet, Old French jaiet "jet, lignite" (12c.), from Latin gagates, from Greek gagates lithos "stone of Gages," town and river in Lycia. As "a deep black color," also as an adjective, attested from mid-15c.