1869, "act or fact of flowing out, a flowing out or forth;" 1875, "that which flows out," from out- + flow (n.).

"flowing like honey, smoothly or sweetly flowing," c. 1600, from French mellifluent and directly from Late Latin mellifluentem (nominative mellifluens), related to mellifluus (see mellifluous). Related: Mellifluence.

early 15c., "sweet as honey, pleasing, sweetly or smoothly flowing" (of an odor, a style of speaking or writing, etc.), from Late Latin mellifluus "flowing with (or as if with) honey," from Latin mel (genitive mellis) "honey" (related to Greek meli "honey;" from PIE root *melit- "honey") + -fluus "flowing," from fluere "to flow" (see fluent). Related: Melifluously; melifluousness.

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.

early 15c., "a flowing back" (of the sea, etc.), "ebb tide," also figurative of instability, from Medieval Latin refluxus, from Latin re- "back, again" (see re-) + fluxus "a flowing," from fluere "to flow" (see fluent). Digestive sense is recorded from 1937; reflux-valve is attested by 1853.