Now there are MANY theories of how the expression came to be. The most common (and likely) are:

1) “pints and quarts” when serving at a bar. It was said that the bartender kept a tally on a blackboard of what he had sold. To tell him to “mind his p’s and q’s” what mean to mind his own business… get back to his work.

problems:

* this suggestion is not found before the early 20th century

* the meaning “mind your own business” is NOT quite what the modern idiom means

2) refers to the forms of the lower case letters p and q — warning for typesetters to be careful not to reverse these two letters, which are easy to confuse, esp since type is set backwards. A variation on this — advice to children to watch out about making the same sort of mistake

I’ve always liked the typesetting solution — it seems extremely convenient that p’s and q’s would be mirror images, and DOES yield the proper sense (of taking great care to get the details straight)

3)) explanation found in Collins Dictionar — it it is an altered form of

“Mind your ‘please’s and ‘thank you’s”.

maybe — sounds rather attractive and DOES have the correct meaning and even the emphasis on polite behavior. Unfortunately, there is no specific evidence of the expression from which it is supposed to be derived (except in explanations of this very question!)

This link discusses these options, favoring #3 above

http://www.yaelf.com/aueFAQ/mifmindyourpsandqs.shtml

See also:

http://www.wordorigins.org/wordorp.htm#mind%20your%20p’s%20&

“In the eighteenth century, at local taverns, pubs, and bars, people drank from pint and quart-sized containers. A bar maid’s job was to keep an eye on the customers and keep the drinks coming. She had to pay close attention and remember who was drinking in ‘pints’ and who was drinking in ‘quarts,’ hence the term: ‘minding your “P’s and Q’s.'”

Other ideas I have discovered concerning the origin of the phrase are as follows:

‘A theory, drawn from the schoolroom, is that any child approaching the mystery of penmanship soon discovers that the lowercase “p” is devilishly easy to confuse with the lowercase “q.” Thus, the theory goes, generations of teachers exhorting their small charges to “mind your P’s and Q’s” created a enduring metaphor for being attentive and careful. A similar theory centers on typesetters in old-fashioned printing shops, where the danger of confusing lowercase “p” and “q” was increased because typesetters had to view the typeset text backwards.

Still other theories tie the “P” to “pea” cloth (the rough fabric used in “pea jackets”) and the “Q” to “queue,” which meant a ponytail, either that of the fancy wigs worn by courtiers of the day or the real ponytails commonly worn by sailors. In the upscale version of this theory, young aristocrats were cautioned not to get the powder from their wigs on their jackets made of pea cloth. The sailor version has old salts advising newcomers to dip their ponytails in tar (a common practice, believe it or not), but to avoid soiling their pea jackets with the tar.’ — The Word Detective

So there are apparently many theories as to how this common saying originated. Pick the one you like the best.

It comes from the old days when printers set type by hand. Sorting the type back into the type bins was the job of the apprentices. The typesetters would admonish the apprentices to pay attention to what they were doing and to get each letter back in its proper little “box.” Typesetters worked very, very fast, moving their hands over the type boxes to pull the letters and spacers they put into place, so they needed to be able to rely on letters being in the correct place.

Two of the hardest to tell apart were the p’s and the q’s. So the typesetters would tell the boys to “mind their p’s and q’s.”

Now days it means to watch what you do (your behavior, usually).