1. a smartass/know it all who think they're cool when they're really pathetic
2. someone who behaves in a retarded/stupid manner
rachel: hey dude have you seen AJ today?
meg: yeah man, he was pissing me off. he's such a rainbow kid.
A hippie like kid who goes to Rainbow gatherings and who usually does not have a home but has a van and usually panhandles for money or sells drugs. A rainbow kid usually hasn't bathed, wears no shoes, and has long hair/dreadlocks.
Q:What's the difference between a rainbow kid and a Phish kid?
A;A phish kid won't pick up a hitchiker because the car is too full of drugs. The rainbow kid is looking for drugs.