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, "scab"
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