while myriad theories exist to explain the origin of the odd little phrase "mind your p's and q's", i've just thought of another one (which seems to make a decent amount of sense, unlike some of the others).
might it be that "p's and q's" is just an eggcorn of "'please and thankyou's" or "'pleases and thankyou's"? the "p's" could be a truncated "please" and the "kyou's" part turns easily into "q's"...
or maybe i just think about language too much.
edit: not the first one to have this thought. :(