One person in a group or gang that is either used, ignored, and/or stepped on quite frequently.
Came into common use with the Buffy the Vampire Slayer Season 3 Episode, "The Zeppo", in which it was highlighted that a particular member of a gang was simply the uncool one. This has various effects on how we use the word today.
The word "Zeppo" is also used in reference to a person in a group who simply doesn't fit, be it because of their lack of coolness or other negative attributes.
Mike was definitely the zeppo; you could tell that he didn't fit in simply by his clothes, attitude, and poise around the others.
A girl who likes to sleep with old codgers.
Zeppo pumped another old yin last night
An intentional peep at another man's testicles while urinating in a bathroom.
While I was in the bathroom the other day, I snuck a quick zeppo at this large african american man.