Derogatory term for a person's head, often used in rough or contemptuous speech. Not a neutral anatomical term.
I smacked that guy's head.
Standard spelling; the そ is written in kana to avoid confusion with other readings of 素.
Ateji spelling; rarely used and may be unfamiliar to modern readers.
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.