a person that is so creepy that words can't describe that person. They usually try to get with girls by staring at their asses from a distance.
Kim: Do you see that weirdo over there?
stranger: Yea hes staring at our ass, what a creep. He might even have an ass fetish.
Kim: Damn that bitch is a withamer, lets peace out.
The age at which one begins to wither. Exactly which age this is varies broadly between individuals, and is subject to genes, environmental and lifestyle factors.