The collar of the shirt is a little dirty.
It was cold, so I turned up the collar of my coat.
nape of the neck; scruff of the neck
Limited or old-fashioned as a standalone body-part word; modern Japanese more often uses うなじ, 首筋, or 襟首 for the nape or scruff.
In older usage, 襟 can refer to the back of the neck, that is, the nape.
It was cold, so I turned up my collar and walked.
He got angry and grabbed the other person by the collar.
The collar of a white shirt gets dirty quickly.
It was hot, so I loosened my collar a little.
scarf; muffler