Some loser who makes multiple empty threats (stabbing toes, boiling teeth etc) insults his chums, and doesn't know how to act right.
"I heard there's a kid out there threatening his friends"
"Jesus Christ, what a rat"
A 'rat' is a traitor, a conceiver, planner or physical participator. He doesn't sell secrets for power or cash. He betrays the trust of his team or his family hoping to save his own cowardly ass.
“The difference is, at least a snitch is human, but a rat is a fuckin' rat, period”
What you say when something is not going how you want it to /not cool.
"Wanna play soccer?"
"Nah"
"Rats"