Fluent

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late 16th century: from Latin fluent- ‘flowing’, from the verb fluere .

wiktionary

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Latin fluens(“flowing”), present active participle of fluō(“I flow”).

etymonline

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fluent (adj.)

1580s, "flowing freely" (of water), also, of speakers, "able and nimble in the use of words," from Latin fluentem (nominative fluens) "lax, relaxed," figuratively "flowing, fluent," present participle of fluere "to flow, stream, run, melt," from extended form of PIE *bhleu- "to swell, well up, overflow" (source also of Latin flumen "river;" Greek phluein "to boil over, bubble up," phlein "to abound"), an extended form of root *bhel- (2) "to blow, swell." Used interchangeably with fluid (adj.) in 17c. in the sense "changeable, not rigid." Related: Fluently.