I can tell you where I first heard the term. A couple of my friends went to school to be physical therapists. In gross anatomy (dissecting a human cadaver) their cadaver was, uhhhhh, excited. So they named him Mr. Happy. The cadaver, not the unit.

The sick SOBs wanted to find out why he was excited after the class was over, so they went to town with the scalpels to find out. Silicon implants. Personally, I thought that the scalpel thing was something you never do, discuss, or even think about, even with a cadaver.