company manners - 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.
One's best behavior, as in George never interrupts when we have guests; he has fine company manners. This term employs company in the sense of “guests.” An older variant, Tell me thy company and I'll tell thee thy manners, uses company in the sense of “companions.” The current term implies that one is more mindful of politeness with invited guests.