Someone who is mean. they are being called mean for whatever reason in a childish manor.
parent: whats wrong Jonnie?
Jonnie: Jack is being a meanie-bo-beenie!
A straight-up, raging asshole/bitch/dick/cunt.
Man, that Ed Kemper guy was a real meanie-bo-beenie.
I don't what he sees in her, she's such a meanie-bo-beenie.