Online Etymology Dictionary

P: 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 see also to be P and Q (1610s), "to be excellent," a slang phrase said to derive from prime quality.
p wave (n.): 1908 in geology, the p representing primary (adj.).
p.a. (n.): abbreviation of "public address" (system), attested from 1936.
P.C.: abbreviation for personal computer is from 1978; abbreviation for politically correct is by 1990.
P.C.P.: also pcp, 1960s, from animal tranquilizer phencyclidine.
P.D.Q.: also pdq, initialism (acronym) for pretty damn quick, attested from 1875.
p.m.: abbreviation of Latin post meridiem "after noon."
p.o.v.: also pov, initialism (acronym) for point of view, by 1973.
P.S.: 1610s, abbreviation of Latin post scriptum (see postscript).
pa: 1804, colloquial shortening of papa (q.v.).
Pablum: See pabulum.
pabulum (n.): "food" for anything, 1670s, from Latin pabulum "fodder, food, nourishment," from PIE root *pa- "to protect, feed" (see food) + instrumentive suffix *-dhlom.

Pablum (1932), derived from this, is a trademark (Mead Johnson & Co.) for a soft, bland cereal used as a food for infants and weak and invalid people, hence figurative use (attested from 1970, first by U.S. Vice President Spiro Agnew) in reference to "mushy" political prose.
paca (n.): Central and South American rodent, 1650s, from Spanish, from Tupi (Brazil) paca.
pace (n.): late 13c., "a step in walking; rate of motion," from Old French pas "a step, pace, trace," and directly from Latin passus, passum "a step, pace, stride," noun use of past participle of pandere "to stretch (the leg), spread out," probably from PIE *pat-no-, a nasalized variant of root *pete- "to spread" (source also of Greek petannynai "to spread out," petalon "a leaf," patane "plate, dish;" Old Norse faðmr "embrace, bosom," Old English fæðm "embrace, bosom, fathom," Old Saxon fathmos "the outstretched arms"). Also, "a measure of five feet" [Johnson]. Pace-setter in fashion is from 1895.
pace (prep.): "with the leave of," 1863, from Latin pace, ablative of pax "peace," as in pace tua "with all deference to you;" from PIE *pak- "to fasten" (see pax). "Used chiefly as a courteous or ironical apology for a contradiction or difference of opinion" [OED].
pace (v.): 1510s, "to walk at a steady rate," from pace (n.). Meaning "to measure by pacing" is from 1570s. That of "to set the pace for" (another) is from 1886. Related: Paced; pacing.
pacemaker (n.): also pace-maker, 1884, originally a rider or boat that sets the pace for others in training. Meaning "the node of the heart which determines the beat rate" is from 1910; sense of "man-made device for stimulating and regulating heartbeat" is from 1951. From pace (n.) + maker.
pachinko (n.): 1953, from Japanese, "pinball machine," also "slingshot, handgun," from pachin, of echoic origin, + diminutive suffix -ko.
pachyderm (n.): 1838, from French pachyderme (c. 1600), adopted as a biological term 1797 by French naturalist Georges Léopole Chrétien Frédéric Dagobert, Baron Cuvier (1769-1832), from Greek pakhydermos "thick-skinned," from pakhys "thick, large, massive," from PIE *bhengh- "thick, fat" (source also of Sanskrit bahu- "much, numerous" Avestan bazah- "height, depth," Hittite pankush "large," Old Norse bingr "heap," Old High German bungo "a bulb," Lithuanian biess "thick") + derma "skin" (see derma).
pachysandra (n.): 1813, from Modern Latin (1803), from Greek pakhys "thick" (see pachyderm) + aner (genitive andros) "man" (see anthropo-), which is used in botany to mean "stamen, having stamens."
pacific (adj.): 1540s, "tending to make peace," from Middle French pacifique, from Latin pacificus "peaceful, peace-making," from pax (genitive pacis) "peace" (see peace) + root of facere "to make" (see factitious). Meaning "peaceful, calm" is first recorded 1630s. Related: Pacifical (mid-15c.); pacifically.
Pacific Ocean: 1660, from Medieval Latin Pacificum, neuter of Latin pacificus (see pacific); so called c. 1500 by Magellan when he sailed into it and found it calmer than the stormy Atlantic.
pacification (n.): "a setting at peace," early 15c., from Middle French pacification "act of making peaceful" (15c.), from Latin pacificationem (nominative pacificatio) "a peace-making," noun of action from past participle stem of pacificare "to pacify" (see pacify).
pacificism (n.): 1904, from pacific + -ism.
pacifier (n.): "one who pacifies or appeases," 1530s, agent noun from pacify. The meaning "nipple-shaped device for babies" is first recorded 1904.
pacifism (n.): 1905, from French pacifisme (by 1903, apparently coined by Émile Arnaud), from pacifique (see pacific).
pacifist (n.): 1903, from French pacifiste (see pacifism). Related: Pacifistic (1902).
pacify (v.): late 15c., "appease, allay the anger of (someone)," from Middle French pacifier "make peace," from Latin pacificare "to make peace; pacify," from pacificus (see pacific). Of countries or regions, "to bring to a condition of calm," c. 1500, from the start with suggestions of submission and terrorization. Related: Pacified; pacifying.
pack (n.): "bundle," early 13c., probably from a Low German word (compare Middle Dutch pac, pack "bundle," Middle Low German pak, Middle Flemish pac, attested from late 12c.), originally a term of wool traders in Flanders; or possibly from Old Norse pakki. All are of unknown origin.

Italian pacco is a Dutch loan word; French pacque probably is from Flemish. Meaning "set of persons" (usually of a low character) is c. 1300, older than sense of "group of hunting animals" (early 15c.). Extended to collective sets of playing cards (1590s), floating ice (1791), cigarettes (1924), and submarines (1943). Meaning "knapsack on a frame" is attested from 1916. Pack of lies first attested 1763.
pack (v.): c. 1300, "to put together in a pack," from pack (n.), possibly influenced by Anglo-French empaker (late 13c.) and Medieval Latin paccare "pack."

Some senses suggesting "make secret arrangement" are from an Elizabethan mispronunciation of pact. Sense of "to carry or convey in a pack" (1805) led to general sense of "to carry in any manner;" hence to pack heat "carry a gun," underworld slang from 1940s; "to be capable of delivering" (a punch, etc.), from 1921. Related: Packed; packing.
pack-horse (n.): late 15c., from pack (n.) + horse (n.).
pack-rat (n.): common name for the North American bushytailed woodrat (Neotoma cinerea) 1885, from pack (v.); so called from the rodents' habit of dragging objects off to their holes. Used figuratively or allusively from c. 1850 of persons who won't discard anything, which means either the rat's name is older than the record or the human sense is the original one.
package (n.): 1530s, "the act of packing," from pack (n.) + -age; or from cognate Dutch pakkage "baggage." The main modern sense of "bundle, parcel" is first attested 1722. Package deal is from 1952.
package (v.): 1915, from package (n.). Related: Packaged; packaging.
packer (n.): mid-14c. (mid-13c. as a surname), agent noun from pack (v.).
packet (n.): mid-15c., from Middle English pak "bundle" (see pack (n.)) + diminutive suffix -et; perhaps modeled on Anglo-French pacquet (Middle French pacquet), which ultimately is a diminutive of Middle Dutch pak. A packet boat (1640s) originally was one that carried mails. Packet-switching attested from 1971.
packsaddle (n.): also pack-saddle, "saddle for supporting packs on the back of a mount," late 14c., pakke sadil; from pack (n.) + saddle (n.).
pact (n.): early 15c., from Old French pacte "agreement, treaty, compact" (14c.), from Latin pactum "agreement, contract, covenant," noun use of neuter past participle of pacisci "to covenant, to agree, make a treaty," from PIE root *pag- "fix, join together, unite, make firm" (source also of Sanskrit pasa- "cord, rope," Avestan pas- "to fetter," Greek pegnynai "to fix, make firm, fast or solid," Latin pangere "to fix, to fasten," Slavonic paž "wooden partition," Old English fegan "to join," fon "to catch seize").
pad (n.): 1550s, "bundle of straw to lie on," possibly from or related to Low German or obsolete Flemish pad "sole of the foot," which is perhaps from PIE *pent- "to tread, go" (see find (v.)), but see path (n.). Meaning "cushion-like part of an animal foot" is from 1790 in English. Generalized sense of "something soft" is from c. 1700; the sense of "a number of sheets fastened together" (in writing pad, drawing pad, etc.) is from 1865.

Sense of "takeoff or landing place for a helicopter" is from 1960. The word persisted in underworld slang from early 18c. in the sense "sleeping place," and was popularized again c. 1959, originally in beatnik speech (later hippie slang) in its original English sense of "place to sleep temporarily."
pad (v.1): "to walk," 1550s, probably from Middle Dutch paden "walk along a path, make a path," from pad, pat "path." Originally criminals' slang, perhaps of imitative origin (sound of feet trudging on a dirt road). Related: Padded; padding.
pad (v.2): "to stuff, increase the amount of," 1827, from pad (n.); transferred to expense accounts, etc. from 1913. Related: Padded; padding. Notion of a padded cell in an asylum or prison is from 1862 (padded room).
padding (n.): "material used in stuffing," 1828, verbal noun from pad (v.2).
paddle (n.): c. 1400, padell "small spade," from Medieval Latin padela, of uncertain origin, perhaps from Latin patella "small pan, little dish, plate," diminutive of patina (see pan (n.)).

Meaning "short oar with a wide blade" is from 1620s. As an instrument used for beating clothes (and slaves, and schoolboys), it is recorded from 1828, American English. Paddle-ball attested from 1935.
paddle (v.1): "to dabble, wade in water," 1520s, probably cognate with Low German paddeln "tramp about," frequentative of padjen "to tramp, to run in short steps," from pad (v.). Related: Paddled; paddling. Meaning "to move in water by means of paddles" is a different word (see paddle (v.3)).
paddle (v.2): "to beat with a paddle, spank," 1856, from paddle (n.). Related: Paddled; paddling.
paddle (v.3): "to move in water by means of paddles," 1670s, from paddle (n.). To paddle one's (own) canoe "do for oneself" is from 1828.
paddle-wheel (n.): also paddlewheel, 1805, from paddle (n.) + wheel (n.).
paddock (n.1): "a frog, a toad," c. 1300, diminutive of pad "toad," from Old Norse padda; common Germanic (Swedish padda, Danish padde, Old Frisian and Middle Dutch padde "frog, toad," also Dutch schildpad "tortoise"), of unknown origin and with no certain cognates outside Germanic.
paddock (n.2): "an enclosure," 1620s, alteration of Middle English parrock, from Old English pearroc "enclosed space, fence" (see park (n.)). Or possibly from Medieval Latin parricus (8c.), which ultimately is from Germanic.
paddy (n.1): "rice field," 1620s, "rice plant," from Malay padi "rice in the straw." Main modern meaning "ground where rice is growing" (1948) is a shortening of paddy field.