Green’s Dictionary of Slang

(US) the penis.

1974		A. Sann Dead Heat 38: Maybe he’s slipping the knockwurst to a rich widow in the Towers [HDAS].
2002		Fathers Canada 14 May posting 🌐 Later, in January 1994, jurors found Mrs. Bobbitt not guilty by reason of insanity on a charge of ‘malicious wounding,’ a legally polite term for lopping off her husband’s ‘knockwurst’.