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Related to tell on: tell off
tell someone on someone
to tattle to someone about someone. I'm going to tell your mother on you! I'll tell the teacher on you!
tell on someone
to report someone's bad behavior; to tattle on someone. If you do that again, I'll tell on you! Please don't tell on me. I'm in enough trouble as it is.
Tattle on, inform on, as in Marjorie said she'd tell on him if he pulled her hair again. This seemingly modern term appeared in a 1539 translation of the Bible (I Samuel 27:11): "David saved neither man nor woman ... for fear (said he) lest they should tell on us."
1. To inform some authority that someone has behaved badly or illegally: The janitor told the teacher on me for writing on the desk. We didn't want to tell on our friends for shoplifting. I promised not to tell on my brother for eating cookies before dinner.
2. To have an effect or impact on someone or something: The stress of working long hours began to tell on the store's owner.