stool on someone
stool on (one)
slang To inform or tattle on one. The accountant was all set to stool on the mob, but they got to him before he made it to the police. Don't stool on your friends, Jack. You've got to have their backs!
stool (on someone)
in. to inform (on someone). (To act as a stool (pigeon).) Britney would stool on anybody, even her own mother.