Think of a hand — or a line of sight — sliding from one end of something to the other, and doing it exactly once. Once is one time; over is travelling across the whole thing. One pass means it is fast, and it means the detail stays where it is. When your eyes make that pass, it is a quick look. When a cloth makes it, it is a quick clean.
可以想成一隻手，或一道目光，從東西的一端滑到另一端，而且只滑這麼一次。once 是一次，over 是從頭到尾經過。只走一次，所以快、所以粗，細節全都留在原地。眼睛這樣走，就是快速看一遍；抹布這樣走，就是隨手擦一遍。兩個意思長得一樣，靠手上有沒有拿東西來分。
From the word’s history
Wiktionary records the phrase as coming from the idea of going over something once. That is still the whole picture today: one pass, end to end, and no second look.
Wiktionary 把這個詞的來源記成「把一件事走過一次」的想法。這條線索直接對上它今天的用法：只過一遍，從頭到尾，沒有第二次。
Wiktionary, "once-over"