by the nape of the neck

by the nape of (one's)/its/the neck

By the back of the neck (of a person or animal). My mom was so mad that she grabbed me by the nape of the neck and hauled me into my room to be grounded. I had to yank the cat out from under the floorboards by the nape of its neck. The boss took John by the nape of his neck and warned him never to do something so foolish again.
See also: by, nape, neck, of
Farlex Dictionary of Idioms.

by the nape of the neck

by the back of the neck. (Mostly said in threats.) He grabbed me by the nape of the neck and told me not to turn around if I valued my life. I stood very still. If you do that again, I'll pick you up by the nape of the neck and throw you out the door.
See also: by, nape, neck, of
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • by the nape of (one's)/its/the neck
  • nape
  • by the scruff of (one's)/its/the neck
  • by the scruff of somebody's/the neck
  • scruff
  • neck and neck
  • stick one's neck out, to
  • take the checkered flag
  • take the chequered flag
  • by a hair
References in classic literature
quick!" Porthos bent over the stair of the mole, and seized by the nape of the neck one of the officers of the royal army who was waiting to embark till all his people should be in the boat.
The Man in the Iron Mask
On the fourth day, lame and sore, scarcely able to see, so closed were his eyes, he was haled from his bunk by the nape of the neck and set to his duty.
The Sea Wolf
He got his teeth into me once, and then I got him by the nape of the neck. Lop-Ear and I sat down, and held the puppies up, and looked at them, and laughed.
Before Adam
So it was, helpless, suspended in the air by the nape of the neck, that he bawled and squalled and choked and coughed till the black, disgusted, flung him down roughly in the canoe's bottom.
Jerry of the Islands
Sat down then, and winked at the lean lady, when I had the high satisfaction of finding him lift me up by the nape of the neck, and fling me over into the pit.
Business Man
It is probably from the dialectal "scuff ", meaning either nape or "scruff " of the neck, hence "to seize or shake by the nape of the neck", or "to strike with an open hand".
Fine words and false theories; MY CITY