a rare letter in the initial position in Germanic, in part because by Grimm's Law PIE p- became Germanic f-; even with the early Latin borrowings in Old English, -p- takes up 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 English vocabulary, and with C and S comprises nearly a third of the dictionary, a testimony to the flood of words that have entered the language since 1066 from Latin, Greek, and French.
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 (1610s), "to be excellent," a slang phrase said to derive from prime quality.