抹了油的那塊布
這一家就是核心本身：材料。
noun · C2
Cloth, usually cotton, that has had oil worked into it so that its surface is smooth and rain cannot soak through. It is used mainly for clothing and covers that have to keep water out.
油布
核心畫面
Hold one picture: a length of cotton with oil pressed into it, so the surface turns smooth and the rain slides off instead of in. Every other meaning is that cloth wearing a different shape — cut it into a coat and the coat takes the cloth's name; add the trousers and the set takes it too. The grammar follows the picture: no article for the material, an for the coat, -s for the whole outfit.
先抓住一張圖：一塊棉布，把油壓進去以後，表面變得滑滑的，雨碰到會滑掉，不會滲進去。其他意思都是同一塊布換了形狀。剪成…更多收起
先抓住一張圖：一塊棉布，把油壓進去以後，表面變得滑滑的，雨碰到會滑掉，不會滲進去。其他意思都是同一塊布換了形狀。剪成一件外套，外套就沿用布的名字。再加上一條褲子，整套也沿用同一個名字。文法剛好跟著這張圖走：講材料時不加冠詞，講外套時加 an，講整套時加 -s。所以看到 oilskin 的時候，冠詞和複數就是在告訴你，現在講的是哪一層。From the word’s history
The source records the word as a compound of oil + skin: cloth given an oiled outer layer that keeps water out the way an animal's skin does. That reading matches the modern core, where the material sense comes first and the clothing senses are named after it.
資料把這個字記成 oil（油）加 skin（皮）的合體：布被抹上油，就多了一層像動物皮一樣擋水的外皮。這條線索正好對上今天的用法——材料的意思排第一，衣服的意思都是跟著布的名字來的。
Wiktionary entry for oilskin, etymology_text: 'From oil + skin.' (source_ref kaikki:3f6f69a190036ae4:807851240:7069)
Two branches
用這塊布做出來的雨具
衣服直接沿用材料的名字，所以差別不在意思，而在文法：a/an 表示只有外套，-s 表示外套加褲子一整套。
noun · C2
A coat made of oilskin, worn to stay dry in heavy rain. This use is countable, so you can say an oilskin, his oilskin, or two oilskins.
油布雨衣（單件）