stool pidgeon
A snitch, tattletale, a rat, basically someone who tells all your business, that one person who will tell the cops on you first chance they get.
Kid A: "Yo, you gotta talk to your girl, man, she's a straight up <i>stool pidgeon</i>."
Kid B: "Why you say that?"
Kid A: "She went and told the cops that I was the one responsible for knocking over mailboxes last weekend."
Kid B: "Why you say that?"
Kid A: "She went and told the cops that I was the one responsible for knocking over mailboxes last weekend."