pi n.

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.

c. 1300 (probably older; piehus "bakery" is attested from late 12c.), "baked dish of pastry filled with a preparation of meats, spices, etc., covered with a thick layer of pastry and baked," from Medieval Latin pie "meat or fish enclosed in pastry" (c. 1300), which is perhaps related to Medieval Latin pia "pie, pastry," also possibly connected with pica "magpie" (see pie (n.2)) on notion of the bird's habit of collecting miscellaneous objects.

According to the OED, the word is not known outside English with the exception of Gaelic pighe, which is from English. In the Middle Ages, a pie had many ingredients, a pastry but one. Fruit pies began to appear c. 1600.

The word in the figurative sense of "something easy" is from 1889; the earlier expression easy as eating pie is by 1884. Slice of the pie in the figurative sense of "something to be shared out" is by 1967. Pie-eyed "drunk" is from 1904. Phrase pie in the sky is attested by 1911, from Joe Hill's Wobbly parody of hymns. Pieman "baker or seller of pies" is by c. 1300 as a surname. Pie chart is from 1922.

type of subatomic particle, 1951, from Greek letter pi + -on.