傷口上結的那層硬殼
這一組是字面的用法，名詞先有，動詞從名詞來。
noun · B1
A hard dry crust that forms over a cut or sore while the skin under it is healing. It is made of blood that has dried, and it drops off by itself once new skin has grown.
痂；結痂
核心畫面
You can think of scab as the rough patch that appears where something has been damaged — a dry, hard, ugly little lid sitting on a surface that used to be whole. It is not a wound and not clean skin; it is the crust in between, and nobody wants to look at it.
可以想成：某個表面破了以後，上面浮出一塊又乾又硬、不太好看的殼。它既不是完好的皮膚，也不是流血的傷口，而是中間那層結…更多收起
可以想成：某個表面破了以後，上面浮出一塊又乾又硬、不太好看的殼。它既不是完好的皮膚，也不是流血的傷口，而是中間那層結出來的東西。這個「難看的一塊」的畫面，是這個字所有用法共同的起點——不管長在皮膚上，還是被拿來罵人。From the word’s history
Scab comes from Old English sceabb and Old Norse skabb, and is related to Latin scabere, to scratch. English shabby is a cousin of it. So the word has always belonged to rough, scratched, unpleasant-looking surfaces.
scab 來自古英文 sceabb 和古北歐語 skabb，跟拉丁文 scabere（抓、刮）同源，英文的 shabby（破舊寒酸）也是親戚。也就是說，這個字從一開始就跟「粗糙的、被刮過的、看起來不體面的表面」綁在一起。這正好解釋了為什麼它既能講傷口上的痂，也能拿來當難聽的罵人字眼。
Wiktionary entry for scab, etymology section (packet source_ref kaikki:3f6f69a190036ae4:571811093:63192)
Two branches
拿來罵人的「難看的一塊」
英文把皮膚上那塊難看的東西拿來罵人，由來已久；後來這個罵法落在罷工時照常上班的人身上。這是概念上的連結，不是說這個意思從醫學義推導出來。
noun · B2
An insulting name for a worker who keeps working, or takes someone else's job, while the other workers at that place have stopped work together to demand better pay or conditions.
工賊（罵人的說法）