late 15c., "in a westerly direction; facing toward the west," from Middle English wester (adj.) "western" (mid-14c.), from Old English westra, variant of westerne (see western) + -ly (2). As an adjective, "coming from the west," 1570s. Contradictory sense of "going to the west" attested by 1630s.[westerly etymology, westerly origin, 英语词源]