taco neck
n.— «On a Tuesday afternoon in March, I woke up with what I call “taco neck.” The muscles on one side of my neck had locked, and my head leaned to the other side, as if I was about to take a bite from a taco.» —“Ending with a question” by George Rede The Oregonian (Portland) June 2, 2008. (source: Double-Tongued Dictionary)
Further reading
In 1968, students at Cheyenne High School in Cheyenne, Wyoming, compiled a collection of their own slang, including the word Clyde, used to refer to one’s head, as in Use your Clyde! This is part of a complete episode.
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