Stand behind something big while the wind is blowing — a hill, a ship's hull, an island. In front of the hill the wind hits hard; behind it the air goes quiet. Leeward is that quiet half of the picture, and the arrow that points into it. Nothing here is fixed to a compass: turn the wind around and the leeward side moves to the other side of the same hill. The word always answers one question, which is where the wind is going, not where north is.
From the word’s history
The dictionary builds leeward from lee, the side away from the wind, plus -ward, the ending that means in that direction (as in forward, homeward). So the word says, in its own parts, towards the sheltered side.
字典把 leeward 拆成 lee 加 -ward。lee 本身就是「背著風的那一邊」，-ward 是方向字尾，跟 forward、homeward 的用法一樣。所以這個字的零件直接拼出「朝背風那一側去」，這正好解釋了為什麼它談的一直是相對於風的位置。
Wiktionary, "leeward"