means the groggy, disoriented exhaustion you feel after flying quickly across several time zones, while your internal clock lags behind the local one

from A 20th-century compound born of the jet age: 'jet' from the jet aircraft that finally made it possible to outrun the sun, plus 'lag' (an old word for falling behind, probably of Scandinavian origin). The word couldn't exist until planes were fast enough to drop you in a new timezone before your body had finished the trip — so it dates from the late 1960s, when commercial jet travel made the affliction common enough to need a name.