Milk and honey be careful of one's behaviour this sounds as if it is a warning to children and it probably originated as a classroom admonition in the days when children learnt to write by copying the letters of the alphabet from a model (see criss-cross and blot one's copybook): p and q were adjacent letters, both had tails and so it would have been easy to confuse the two. A teacher's catchphrase advising care and correctness in writing might readily have become generally adopted as one advocating similar virtues in behaviour. A more fanciful suggestion is that p's and q's were abbreviations of the pints and quarts recorded on a blackboard by a publican keeping a tally of a customer's drinking (see chalk up). The expression then becomes a customer's warning to the innkeeper to get the sums right.