An ass-snitch is someone who is an asshole and a snitch, meaning, they snitch on you, but lie about what they are snitching on you for, resulting in you getting in more trouble.
Victim of ass-snitching: "I spilled my soda behind the couch, but I cleaned it up."
Ass-snitcher: *Runs to a parent* "Hey! Mom, he poured his soda behind the couch!"
Victim of ass-snitching: "You ass-snitch! I cleaned it up, and it spilled on accident..."
Ass-snitcher: "He's lying, I had to clean it up."