Do you know ghouls from goblins and ghosts?
early 15c., "a flowing back (of the sea, etc.)," from Medieval Latin refluxus, from Latin re- "back, again" (see re-) + fluxus "a flowing" (see flux). Digestive sense is recorded from 1937.
reflux re·flux (rē'flŭks')n.
A flowing back.
The process of refluxing.