Fluid

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late Middle English (as an adjective): from French fluide or Latin fluidus, from fluere ‘to flow’.

wiktionary

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From Middle English fluid, from Latin fluidus(“flowing; fluid”), from Latin fluō(“to flow”), from Proto-Indo-European *bʰleh₁-(“to swell; surge; overflow; run”). Akin to Ancient Greek φλύειν(phlúein, “to swell; overflow”). Not related to English flow, which is a native, inherited word from *plew-.

etymonline

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fluid (adj.)

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.

fluid (n.)

"substance capable of flowing," 1660s, from fluid (adj.). Related: Fluidal (1869), fluidic (1821, Marmaduke Tulket).