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Old English fleot "ship, raft, floating vessel," from fleotan "to float" (see fleet (v.)). Sense of "naval force" is pre-1200. The Old English word also meant "creek, inlet, flow of water," especially one into the Thames near Ludgate Hill, which lent its name to Fleet Street (home of newspaper and magazine houses, standing for "the English press" since 1882), Fleet prison, etc.
"swift," 1520s, but probably older than the record; apparently from or cognate with Old Norse fliotr "swift," and from the root of fleet (v.)). Related: Fleetness.
Old English fleotan "to float, drift, flow, swim, sail," later (c.1200) "to flow," from Proto-Germanic *fleut- (cf. Old Frisian fliata, Old Saxon fliotan "to flow," Old High German fliozzan "to float, flow," German flieszen "to flow," Old Norse fliota "to float, flow"), from PIE root *pleu- "to flow, run, swim" (see pluvial).
Meaning "to glide away like a stream, vanish imperceptibly" is from c.1200; hence "to fade, to vanish" (1570s). Related: Fleeted; fleeting.