sixteenth letter of the English alphabet, descended from the Greek pi; the form of it is a pi with the second limb curved around to meet the first. A rare letter in the initial position in Germanic, in part because by Grimm's Law PIE p- became Germanic f-; even including the early Latin borrowings in Old English, "P" has 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 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, especially those in pre- and pro-.
Between -m- and another consonant, an unetymological -p- sometimes is inserted (Hampstead, Thompson) to indicate that the -m- is sounded as in words such as Simpson. 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 see also to be P and Q (1610s), "to be excellent," a slang or provincial phrase said to derive from prime quality.
P-wave is from 1908 in geology, the p representing primary (adj.). The U.S. Navy World War II PT boat (1942) stands for patrol torpedo.