By the scruff of the/somebody's neck phrase
If you say that you take, hold, grab,etc. an animal or a person by the scruff of their neck, you mean that you take, hold, grab,etc. by the back of their neck.
I took the puppy out of the mess by the scruff of its neck.
The security staff took him by the scruff of his neck and threw him out.
I wonder why mother cats pick kittens up by the scruff of the neck.
The origin of this phrase is not clear.