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.
"pertaining to a river," late 14c., from Latin fluvialis "of a river," from fluvius "a river, stream, running water," related to fluere "to flow" (see fluent).
late 12c., "stream," from Old French flum "running water, stream, river; dysentery," from Latin flumen "flood, stream, running water," from fluere "to flow" (see fluent). In U.S., used especially of artificial streams channeled for some industrial purpose.
early 15c., "liquid, capable of flowing," from Old French fluide (14c.) and directly from Latin fluidus "fluid, flowing, moist," from fluere "to flow" (see fluent). Figurative use, of non-material things, "not fixed or rigid," from 1640s. Related: Fluidly.
1630s, from Latin fluctuatus, past participle of fluctuare "to undulate, to move in waves," from fluctus "a wave, billow, surge, a flowing," from past participle of fluere "to flow" (see fluent). Related: Fluctuated; fluctuates; fluctuating.