The act of inserting a penis inside of another man's urethra for sexual pleasure.
Whenever a medical procedure is done where the patient is conscious, someone will develop a fetish for it. In the 1800s, the treatment for syphilis was mercury and urethral sounds, or large metal probes stuck up the pee hole. A few patients developed a fetish for this, and that is where 'sounding' comes from. Eventually, a gay man had a fetish for sounding, and had a partner with a needle dick. One thing leads to another, and the concept of cockstuffing was stolen from the gods.