Picture an arrow drawn on a map with east at one end of it. Easterly is the word for everything that arrow touches: the way a road runs, the side of a lake it points at, the path a ship takes. There is one twist worth holding on to. When the thing being described is wind, English names it by where it starts, so the arrow flips: an easterly wind leaves the east and travels away from it.
From the word’s history
The source records easterly as easter (east) plus -ly. That -ly is the sense of belonging to a side rather than the adverb-forming one, which is why northerly, westerly and southerly are all built the same way.
資料記的是：easterly 由 easter（東）加 -ly 構成。這裡的 -ly 可以想成「屬於那一邊的」，而不是變副詞的那個 -ly；northerly、westerly、southerly 也都是同一套做法。
Wiktionary (kaikki:3f6f69a190036ae4:1113783652:8056), etymology of easterly (adj)
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