I wouldn’t ask Cecil. I’d ask Pat:
I found no reference to Ps & Qs in that link. (Did I do something wrong?)
The grammarphobia article: Mind your p’s and q’s
Uncle Cecil’s article: What’s the origin of the expression “mind your P’s and Q’s”?
Cecil doesn’t do much etymology nowadays, since the advent of the internet and so many websited dedicated to word/phrase origins. However, in his column (from 1989) he cites an OED reference from 1612 (spelled “Pee and Kew.”) Ms O’Conner on the Grammarphobia blog cites 1779, also from the OED.
I don’t have an OED to check them, but 'praps some kind soul will do so here. Frankly, I find Cecil’s article more complete and more informative, but I guess it’s a matter of taste.
In short, no definitive answer. It’s likely that several expressions were influenced by one another or melded together in common speech.
Cecil’s reference is OK, but, although Hannah Cowley is real, and Who’s the Dupe is a real play that she produced in 1779, it is not cited in the OED3 s.v. p’s and q’s
I’m a little surprised by the answer that q.t. is an abbreviation for quiet.
I’d like to point out that there is at least one famous latin expression in which ‘he who is silent’ forms a part.
qui tacet consentire–he who is silent is take to agree.
Therefore, there may be an equivalent expression like “qui tacet”-- that means “he/it who/which is silent.”
While the declension may be incorrect (It may be qui tacere or something) I think it’s a more likely answer for Q.T. than an abbreviation for quiet.
Q.T. is probably an old legal, parliamentary, or literary expression (like q.v.) that has passed from use.
I for one think ‘quiet’ would be abbreviated as qt. Instead whenever I’ve seen the reference it’s always Q.T., which usually indicates an abbreviation of a Latin expression.
I would check with a Classics professor.
Etymonline again:
All the word origin sites say the same thing, with more detail. None suggests any connection to the Latin phrase.
This was a Just Saying thing. I emailed Pat twice and she answered both times. I’ve never really used her site.
We had a nice conversation on the word–the spoken word that I detest whenever I hear it:
Empathetic. (Empathy.) Drives me crazy. What the hell is wrong with empathic? Who turned it into a version of emphasize or pathetic.
This is in my hit parade of pet peeves.
But it makes no sense. “We want to do this on the he who is silent.” Bleah! Indeed, all the early citations in the OED are “strict” or “strictly q.t.”, which makes even less sense. And it’s not as though the word itself is obscure—it’s given us the English word “tacit”, and any performer knows, as well, that “tacet” is the musical equivalent of “This space intentionally left blank.” The OED says it’s “q[uie]t” (which used to be a common style of abbreviation, as in “M[iste]r” or “M[ain]e”), and I’d need some hard proof before I’d disagree.
Yet you don’t complain that it’s ‘sympathetic’ instead of ‘sympathic’?
Besides, using “empathetic” frees up “empathic” for the sci-fi use. =)
Powers &8^]
The emphasis is on the empathy?
For some reason I’ve always thought that “mind your P’s and Q’s,” meant be sure to say please and thank you. It’s a play on the similar sounding between Q and “thank you”. I don’t know how I got this idea, but I find it surprising how different it is from Cecil’s explanations.
http://www.straightdope.com/columns/read/761/whats-the-origin-of-the-expression-mind-your-ps-and-qs
Welcome to the Straight Dope Message Boards, Sloth4z, glad to have you with us. And thanks for providing a link to the column!
Since there was already a thread on this topic, I’m merging your new thread into that one. Keeps all discussion on P’s and Q’s in the same place.
On your comment: Agreed, there’s a pun on Q’s but there’s also that P’s sounds like a child’s rendition of “Please” (omitting the difficult L sound.) Attractive theory, but not mentioned by most etymologists. 
In logic, the letters p and q are typically used in the same way that x and y are used in algebra. Perhaps that has something to do with it?
No, the expression is much older than modern formalizations of logic. If that had been the case, the system (in reference to the old rules of Barbara, Celarent, etc.) might have turned out something like *“Mind your e’s and i’s.”
I just remembered something. I read somewhere that it had to do with the olden way of setting type–one letter at a time. And the shop owner would remind the typesetters not to mix them up, the p’s and the q’s) when putting them away.
FWIW
Did you perhaps read it in Cecil’s column?
Which is, you know, the subject under discussion here?
Powers &8^]
Cecil who? Or nope. 
I, for one, certainly encountered the typesetting argument long before Cecil wrote his first column. (And, deep in my past, I’ve actually used a composing stick and a California box.)