Green’s Dictionary of Slang

1. (US) a large breast.

1971

T. Thackrey Thief 304: Those knockers would have been the first thing you noticed on any other broad – real Hermans!

2. (US campus) the penis.

1991		Eble Campus Sl. Fall.
1996		Eble Sl. and Sociability 128: ‘Penis’ has many synonyms, including Herman the one-eyed German.
2002		The Drake & Zeke Thing Rock103.com 🌐 409 WAYS OF TAKIN HERMAN TO THE CIRCUS [...] 123. Jack ’n the beanstalk.