Origin of the word P

1. a rare letter in the initial position in Gmc., in part because by Grimm's Law PIE p- became Gmc. f-; even with early L. borrowings, -p- takes up only a little over 4 pages in J.R. Clark Hall's "Concise Anglo-Saxon Dictionary," compared to 31 pages for B and more than 36 for F. But it now is the third most common initial letter in the Eng. vocabulary, and with C and S, comprises nearly a third of the words in the dictionary, a testimonial to the flood of words that have entered the language since 1066 from L., Gk., and Fr. To mind one's Ps and Qs (1779), possibly is from confusion of these letters among children learning to write. Another theory traces it to old-time tavern-keepers tracking their patrons' bar tabs in pints and quarts. But cf. also to be P and Q (1612), "to be excellent," a slang phrase said to derive from prime quality.

via Online Etymology Dictionary, ©2001 Douglas Harper