The male genitals: stay away from the crown jewels
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with crown jewels
A prized possession or asset, as in The Iliad and Odyssey are the crown jewels of ancient literature, or The software products are the company's crown jewels. This usage transfers the value of royal jewels to some other object.
[ Late 1800s
family jewels. The male genitals, especially the testicles. For example, She gave the would-be mugger a hard kick in the family jewels. A slang euphemism, the term dates from the 1970s, and the variant from the early 1900s.