"Well, his dick is probably impotent, so I understand..."
The best line I heard in this vein is "san senchi go miri" ("3 centimeters, 5 millimeters" 3.5cm. About an inch and a half)
When this old fart was making an ass of himself at a karaoke bar, the skipper of the boat I was sailing turned to him and said, "Omae, san senchi go miri daro". It was outrageous!