yellow gumball
1) out of a group of siblings, the kid who is clearly liked the least, especially by the parents
2) the worst flavor of gumball, usually some vile combination of banana and lemon, that everyone tries to avoid
kid 1: did you hear Bethany's brother got an iPhone 5 AND a jetski for his tenth birthday?
kid 2: what? Bethany doesn't even have a Razor--and she's graduating!
kid 1: i know, she's definitely the yellow gumball.

kid 1: ooh, what flavor did you get?
kid 2 (dejectedly): yellow.
kid 1: oh. gross.