The Girth Test is a form of measuring the girth of, or circumference of, a man's penis. When you run out of toilet paper and begin to jerk off, you stick the empty tube on your penis. If the tube fits and your cock slides right in, you have failed. If you can't fit, you passed.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.