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1.
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.
by Nick March 27, 2005
 
2.
A girl who likes to sleep with old codgers.
Zeppo pumped another old yin last night
by Boab's jokes June 21, 2010
 
3.
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.
by Adanis March 16, 2005