P's & Q's

[Postdog]

as in "mind your". Can anyone explain it?

[andy-hughes]

Opinions as to the origin of this phrase vary -

one explanation is that the written letters loook similar, and can be confused if not written clearly,

another is the theory that it's a shortened version of the old inn-keeper's shout "Mind your pints and quarts!" when yet another bout of fighting broke out.

[shaneystar2]

According to World Wide Words
Advice to a child learning its letters to be careful not to mix up the handwritten lower-case letters p and q.
Similar advice to a printer�s apprentice, for whom the backward-facing metal type letters would be especially confusing.
Jocular, or perhaps deadly serious, advice to a barman not to confuse the letters p and q on the tally slate, on which the letters stood for the pints and quarts consumed �on tick� by the patrons.
An abbreviation of mind your please�s and thank-you�s.
Instructions from a French dancing master to be sure to perform the dance figures pieds and queues accurately.
An admonishment to seamen not to soil their navy pea-jackets with their tarred queues, that is, their pigtails.

A full explanation here
http://www.worldwidewords.org/articles/psandqs.htm