This may be what is known as an 'off-color joke'.
...it's an oldie that's making the rounds again...it's still funny.
A very overweight man was waiting in the examination room at a doctors office.
The doctor walked in and looking over the huge naked fellow said: "So, what seems to be the problem?"
The overweight man said: "It's my weight Doc! For One thing, I've been this size for so long I haven't seen my penis in years!"
The good doctor replied: "Well Sir, why don't you diet?"
The big guy cried out in horror: "Dye it? My God Doc. What color is it now?"