It forms all or part of: fletcher; fledge; flee; fleet (adj.) "swift;" fleet (n.2) "group of ships under one command;" fleet (v.) "to float, drift; flow, run;" fleeting; flight (n.1) "act of flying;" flight (n.2) "act of fleeing;" flit; float; flood; flotsam; flotilla; flow; flue; flugelhorn; fluster; flutter; fly (v.1) "move through the air with wings;" fly (n.) "winged insect;" fowl; plover; Pluto; plutocracy; pluvial; pneumo-; pneumonia; pneumonic; pulmonary.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit plavate "navigates, swims;" Greek plynein "to wash," plein "to navigate," ploein "to float, swim," plotos "floating, navigable," pyelos "trough, basin;" Latin plovere "to rain," pluvius "rainy;" Armenian luanam "I wash;" Old English flowan "to flow;" Old Church Slavonic plovo "to flow, navigate;" Lithuanian pilu, pilti "to pour out," plauju, plauti "to swim, rinse."
1590s, "disturbance, commotion," a word of unknown origin. Meaning "mental trouble" is from 1640s; verb sense of "to fluster" is attested from 1690s. According to OED originally rhyming with other, brother; the pronunciation shift came in 19c. by influence of bother.
c. 1300 (intransitive), "To make a quick sharp noise with frequent repetitions and collisions of bodies not very sonorous: when bodies are sonorous, it is called jingling" [Johnson]. Perhaps in Old English but not recorded; if not, from Middle Dutch ratelen, which is probably of imitative origin (compare German rasseln "to rattle," Greek kradao "I rattle").
The sense of "utter smartly and rapidly, speak with noisy and rapid utterance" is attested by late 14c. The meaning "to go along loosely and noisily" is from 1550s. The transitive sense is from late 14c. The colloquial American English figurative meaning of "fluster, shake up, unsettle" is attested by 1869, on the notion of "startle or stir up by noisy means." Related: Rattled; rattling.