Frenum Piercing
The piercing of one's Frenum or Frenulum, located at the underside of the penis where (if uncircumsized) the foreskin attaches to the head of the penis. If circumcised it's directly under the head of the penis, where the head meets the shaft.

This type of piercing is second in popularity to the Prince Albert.
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     A Frenum Piercing would be right there