Why is the ninth month called September?
1590s, "to run or bend in a certain direction" (of rivers, coasts, etc.), from Middle English trenden "to roll about, turn, revolve," from Old English trendan, from Proto-Germanic *trandijanan (cf. Old English trinde "round lump, ball," Old Frisian trind, Middle Low German trint "round," Middle Low German trent "ring, boundary," Dutch trent "circumference," Danish trind "round"); origin and connections outside Germanic uncertain. Sense of "have a general tendency" (used of events, opinions, etc.) is first recorded 1863, from the nautical sense. Related: Trended; trending.
"the way something bends" (coastline, mountain range, etc.), 1777, from trend (v.); sense of "general tendency" is from 1884.