keep a civil tongue in (one's) head
To speak kindly and politely. Please try to keep a civil tongue in your head the next time you talk to Mary, instead of arguing with her, OK?
keep a civil tongue in one's head
Speak politely, as in The teacher won't allow swearing; she says we must keep a civil tongue in our heads. This expression uses tongue in the sense of "a manner of speaking," a usage dating from the 1400s. An early cautionary version was "Keep a good tongue in your head, lest it hurt your teeth" (1595).
keep a civil tongue in your head
speak politely and calmly, without rudeness.