What is the X in X-mas?
principal river in western Germany, from German Rhein, from Middle High German Rin, ultimately from Gaulish Renos, literally "that which flows," from PIE root *reie- "to move, flow, run" (cf. Sanskrit rinati "causes to flow," ritih "stream, course;" Latin rivus "stream;" Old Church Slavonic reka "river;" Middle Irish rian "river, way;" Gothic rinnan "run, flow," rinno "brook;" Middle Low German ride "brook;" Old English riþ "stream;" Old English rinnan, Old Norse rinna "to run," Dutch ril "running stream"). The spelling with -h- (cf. Latin Rhenus; French Rhin) is from influence of the Greek form of the name, Rhenos.