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