mind one's own business - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Keep from meddling, pay attention to one's own affairs, as in If she would only mind her own business, there would be a lot fewer family quarrels. Already described as a wise course by the ancients (Seneca had it as Semper meum negotium ago, “I always mind my own business”), this precept has been repeated in English since about 1600.