fleeting (adj.)youdaoicibaDictYouDict[fleeting 词源字典]
early 13c., "fickle, shifting, unstable," from Old English fleotende "floating, drifting," later "flying, moving swiftly," from present participle of fleotan "to float, drift, flow" (see fleet (v.)). Meaning "existing only briefly" is from 1560s. Related: Fleetingly.[fleeting etymology, fleeting origin, 英语词源]