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.
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)
Two branches
往東・靠東（箭頭指向東邊）
這一組就是核心畫面的正面用法：箭頭指著東邊，於是路線、位置、走向都跟著往東。
adjective · B2
Going towards the east, facing the east, or sitting on the east side of something. An easterly direction points east, and the easterly end of a lake is the end nearer the east.
朝東的；東邊的
從東邊來的風（箭頭翻過來）
還是同一條東西向的箭頭，但英文替風命名時看的是起點，所以方向感整個翻了一面。這是一條用法上的規則，不是說這個意思在歷史上比較晚才出現。
adjective · B2
An easterly wind blows from the east. Used of wind, the word names the place the wind starts from, not the place it is heading to.
（風）從東邊來的