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.
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Harper, D. (n.d.). Etymology of fluidity. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/fluidity