這個拼法底下其實是2個不同的字，意思沒有關係， 只是長得一樣。下面分開列出來。跳到 揪住後頸 · 邋遢的人
One way to hold this side of the word: the fold of loose skin at the back of an animal's neck, the handful a mother cat takes in her teeth. Everything here is one small grip that controls a whole body — the body part itself, the fixed phrase for holding someone there, and the verb made from the action.
This scruff is recorded from 1790, from an earlier scuft (1787) and dialect scuff. It is related to North Frisian skuft and Dutch schoft, both meaning the back of a horse's neck or its withers, so the neck was in the word from the start.
這個 scruff 從 1790 年左右開始用，更早的形式是 1787 年的 scuft，方言裡寫作 scuff。它跟北弗里斯蘭文 skuft、荷蘭文 schoft 有關，那兩個字指的都是馬的頸背。所以「脖子後面那一塊」從一開始就在字裡面，不是後來才引申出來的。
Wiktionary via kaikki snapshot 3f6f69a190036ae4, scruff etymology 2: 1790, from earlier (1787) scuft and dialect scuff; related to North Frisian skuft, Dutch schoft (withers of a horse)
From the picture
後頸那一把
三個用法都繞著同一塊皮打轉：先是部位，再是抓住它的說法，最後直接變成動作。
noun · C1
If you hold or take a person or an animal by the scruff of the neck, you grip the loose skin, the fur, or the clothing at the back of their neck, so that they cannot easily pull away or turn round.
揪住後頸
noun · C1
The loose skin at the back of the neck on some animals, which a mother animal can take in her teeth to carry her young. The back of a person's neck is sometimes called this too, though that use is much less common.
後頸鬆皮
A second word, same spelling
One way to hold this side of the word: a surface with a rough, uncleaned layer sitting on it. The dictionary sends this scruff back to scurf, the flakes and crust on skin. From that you can see the two living uses: the short rough hair a man has not shaved off, and the person whose whole appearance looks as if nobody has cleaned it up.
Wiktionary files this scruff under etymology 1 with the note: see scurf. Scurf is the flaky crust that forms on skin, so the older sense of this word was a crust or layer on a surface.
Wiktionary 把這個 scruff 放在第一條來源底下，註記只有一句：見 scurf。scurf 是皮膚上剝落的碎屑或結痂，所以這個字早期說的是覆在表面的一層東西。今天的「鬍渣」和「邋遢的人」都可以從這個畫面理解。
Wiktionary via kaikki snapshot 3f6f69a190036ae4, scruff noun etymology 1: See scurf. (also senses tagged obsolete: Crust; Scurf)
From the picture
表面沒整理過的那一層
這一組把「沒清掉的一層」放在人身上：先是臉上真的長出來的毛，再擴大成整個人給人的印象。
noun · C1
A person who looks untidy or dirty, whose clothes, hair and shoes all look as if no one has taken care of them.
邋遢的人
noun · C1
Short rough hair on a man's face, the kind that grows in a few days when he does not shave.
短鬍渣