When should I use "Keep it civil"?
Here is where I first heard the slang: Champion for Democracy? - Woodrow Wilson
Towards the end of the video, Neidell urges viewers to post their views about Wilson and says: "Please keep it civil!", in a seemingly cautioning, still friendly, manner.
I looked up its definition and as per Urban Dictionary, I concluded it's best suited for a heated discussion or a raw, not as friendly advice.
The question is: When should I use it? Does it reflect anger or intimidation? Could it be used in a friendly manner? And is it old-fashioned?
Sometimes said as "keep a civil tongue" instead of "keep it civil," this phrase can be used as a warning to someone/ a group to maintain civil (courteous and polite) behavior, usually when a controversial topic will be discussed or you see that tensions are rising between people for any reason.
Using "keep it civil" can elicit different reactions depending on how you say it.
E.g.) Scolding someone with this phrase could lead to them actually acting even less civil.
E.g.) Using it casually when addressing 2+ people would probably be an effective reminder to them.
As for it being old, I disagree. It's something I still use today pretty commonly.
Here's the TFD definition of keep a civil tongue
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?
The more common idiomatic expression is keep a civil tongue (in one's head):
Fig. to speak decently and politely.
- Please, John, don't talk like that. Keep a civil tongue in your head. John seems unable to keep a civil tongue.*
(McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.)
As noted, the expression is fairly common.
Civil:
The sense of "polite" was in classical Latin, but English did not pick up this nuance of the word until late 16c., and it has tended to descend in meaning to "meeting minimum standards of courtesy." "Courteous is thus more commonly said of superiors, civil of inferiors, since it implies or suggests the possibility of incivility or rudeness" [OED].
(Etymonline)
While the connection to "keep a civil tongue in your head" is clear, there is a larger, and somewhat less personal aspect.
The operational term here is civility
https://www.google.com/search?q=civility&oq=civility&aqs=chrome..69i57j0l5.1487j1j8&sourceid=chrome&ie=UTF-8
formal politeness and courtesy in behavior or speech. "I hope we can treat each other with civility and respect"
Or, less formally, https://www.dictionary.com/browse/civility?s=t
courtesy; politeness.
So, to keep it civil means to maintain civility, to keep the conversation polite.
The injunction "keep a civil tongue in your head" is rather rude and expresses an arrogance in that it assumes that the speaker has the right to curtly demand a certain behavior from the person being addressed. As such it is itself borderline uncivil.