noun
Derogatory term for a person's head, often used in rough or contemptuous speech. Not a neutral anatomical term.
あいつのそ首をはたいてやった。
I smacked that guy's head.
Neutral word for 'head'. そ首 is derogatory and rough.
The derivation is uncertain. The そ may be a prefix of uncertain origin, and 首 means 'neck' or 'head'. The whole word is used as a derogatory term for the head.