jet lag (n.)

1690s, "stream of water," from French jet "a throw, a cast; a gush, spurt (of water); a shoot (of a plant)," from jeter "to throw, thrust" (from PIE root *ye- "to throw, impel"). Middle English had jet/get "a device, mode, manner, fashion, style" (early 14c.).

Sense of "spout or nozzle for emitting water, gas, fuel, etc." is from 1825. Hence jet propulsion (1855, originally in reference to water) and the noun meaning "airplane driven by jet propulsion" (1944, from jet engine, 1943). The first one in service was the German Messerschmitt Me 262. Jet set first attested 1951, shortly before jet commuter plane flights began. Jet age is attested from 1952. The atmospheric jet stream is from 1947.

in the mechanical sense "retardation of movement," 1855, from lag (v.). Also noted in Farmer and Henley ("Slang and Its Analogues") as American theatrical slang for "a wait," with an attestation from 1847. First record of lag time is from 1951.