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.

the thing that you don’t have when you’re phones percentage is over 20

A girl who likes to sleep with old codgers.

An intentional peep at another man's testicles while urinating in a bathroom.