Z
not a native letter in Old English; in Anglo-French words it represents the "ts" sound (as in Anglo-French fiz, from Latin filius, modern Fitz); from late 13c. it began to be used for the voiced "s" sound and had fully taken that role by 1400. For letter name, see zed.
Thou whoreson Zed, thou vnnecessary Letter. ["King Lear," II.ii.69]
Series of zs to represent a buzzing sound first attested 1852; zees "spell of sleep, a nap" is slang first recorded 1963, American English student slang.
za (n.)
U.S. student slang shortening of pizza, attested from 1968.
Zacchaeus
masc. proper name, from Late Latin Zacchaeus, from Greek Zakkhaios, from Hebrew zakkay, literally "pure, innocent," from zakhah "was clean, was pure."
Zachariah
masc. proper name, Late Latin Zacharias, from Greek Zakharias, from Hebrew Zekharyahu, literally "the Lord has remembered," from zakhar "he remembered."
zaftig (adj.)
"alluringly plump, curvaceous, buxom," 1937, from Yiddish zaftik, literally "juicy," from zaft "juice," from Middle High German saft "juice" (see sap (n.1)).
zag (v.)
1793, from zig-zag (q.v.).
zaibatsu
1937, from Japanese zaibatsu, from zai "wealth" + batzu "clique."
Zaire
African nation (1971-1997), from an early alternative name of the Congo River, from Kikongo nzai, dialectal form of nzadi "river."
zakat (n.)
obligatory Islamic tax for religious purposes, 1802, from Persian zakat, etc., from Arabic zakah.
Zamboni (n.)
proprietary name of a machine used to resurface ice skating rinks, 1957, trademark of Frank J. Zamboni & Co., Paramount, Calif.
zany (n.)
comic performer, 1580s, from French zani, from Italian zani, zanni "a zany, clown," originally Zanni, Venetian dialect variant of Gianni, pet form of Giovanni "John;" thus equivalent to English Jack. A stock character in old comedies, he aped the principal actors.
zany (adj.)
1610s, from zany (n.). Related: Zanily; zaniness.
Zanzibar
island off East Africa, from Zengi, name of a local people, said to mean "black," + Arabic barr "coast, shore." Related: Zanzibari.
zap
1929 as a sound, 1942 as a verb; a comic strip word (especially from "Buck Rogers in the Twenty-Fifth Century"), of imitative origin. Meaning "erase electronically" is 1982. Related: Zapped; zapping.
zapper (n.)
electrical pest-killer, 1970, from zap.
Zarathustra
from Avestan Zarathushtra (see Zoroastrian). Related: Zarathustrian.
zarf (n.)
"holder for a coffee cup," 1836, from Arabic zarf "vessel."
zeal (n.)
"passionate ardor in pursuit of an objective or course of action," late 14c., from Old French zel (Modern French zèle) and directly from Late Latin zelus "zeal, emulation" (source also of Italian zelo, Spanish celo), a Church word, from Greek zelos "ardor, eager rivalry, emulation," "a noble passion" [Liddell & Scott], but also "jealousy;" prom PIE *ya- "to seek, request, desire." From mid-15c. as "devotion."
zealot (n.)
early 14c., "member of a militant 1st century Jewish sect which fiercely resisted the Romans in Palestine," from Late Latin zelotes, from Greek zelotes "one who is a zealous follower," from zeloun "to be zealous," from zelos "zeal" (see zeal). Extended sense of "a fanatical enthusiast" first recorded 1630s (earlier in this sense was zelator, mid-15c.).
zealotry (n.)
"excessive or undue zeal, fanaticism," 1650s, from zealot + -ry.
zealous (adj.)
1520s, from Medieval Latin zelosus "full of zeal" (source of Italian zeloso, Spanish celoso), from zelus (see zeal). Related: Zealously, zealousness.
zebra (n.)
c.1600, from Italian zebra, perhaps via Portuguese, earlier applied to a now-extinct wild ass, of uncertain origin, said to be Congolese [OED], or Amharic [Klein], but perhaps ultimately from Latin equiferus "wild horse," from equus "horse" (see equine) + ferus (see fierce). Related: Zebrine; zebroid.
zebu (n.)
Asiatic ox, 1774, from French zebu, ultimately of Tibetan origin. First shown in Europe at the Paris fair of 1752.
Zebulon
masc. proper name, Biblical son of Jacob by Leah, from Hebrew Zebhulun, from zebhul "a dwelling" + diminutive suffix -on (see Gen. xxx:20).
Zechariah
masc. proper name, Biblical 11th of the Twelve Prophets; see Zachariah.
zed (n.)
c.1400, from Middle French zede, from Late Latin zeta, from Greek zeta, from Hebrew zayin, letter name, literally "weapon;" so called in reference to the shape of this letter in ancient Hebrew. U.S. pronunciation zee is first attested 1670s. Other dialectal names for the letter are izzard, ezod, uzzard, and zod.
zee (n.)
"the letter Z," 1670s, now more common in American English.
zein (n.)
simple protein obtained from maize and wheat, 1822, from zea, Late Latin name for "spelt," from Greek zeia "one-seeded wheat, barley, corn" (from PIE root *yewo-) + -in (2).
Zeiss (adj.)
in reference to spy-glasses or binoculars, 1905, from the firm founded by German optical instrument manufacturer Carl Zeiss (1816-1888).
zeitgeist (n.)
1848, from German Zeitgeist (Herder, 1769), "spirit of the age," literally "time-spirit," from Zeit "time" (see tide (n.)) + Geist "spirit" (see ghost (n.)). Carlyle has it as a German word in "Sartor Resartus" (1840) and translates it as "Time-Spirit."
zek (n.)
"Russian condemned person in a prison or labor camp," 1968, from Russian zek, probably representing a vocalization of z/k, abbreviation of zaklyuchennyi "prisoner."
Zen (n.)
school of Mahayana Buddhism, 1727, from Japanese, from Chinese ch'an, ultimately from Sanskrit dhyana "thought, meditation," from PIE root *dheie- "to see, look" (cognates: Greek sema "sign, mark, token"). As an adjective from 1881.
Zend (n.)
1715, "Parsee sacred book" (in full, Zend-Avesta, 1620s), from Old Persian zend, from Pahlavi zand "commentary." First used 1771 in reference to the language of the Zend-Avesta by French scholar Abraham Hyacinthe Anquetil-Duperron (1731-1805).
zenith (n.)
"point of the heavens directly overhead at any place," late 14c., from Old French cenith (Modern French zénith), from Medieval Latin cenit, senit, bungled scribal transliterations of Arabic samt "road, path," abbreviation of samt ar-ras, literally "the way over the head." Letter -m- misread as -ni-.

The Medieval Latin word could as well be influenced by the rough agreement of the Arabic term with classical Latin semita "sidetrack, side path" (notion of "thing going off to the side"), from se- "apart" + *mi-ta-, suffixed zero-grade form of PIE root *mei- (1) "to change" (see mutable). Figurative sense of "highest point or state" is from c.1600.
zeno-
late 20c. word-forming element used in reference to the planet Jupiter, from Greek zeno-, comb. form from Zeus (see Zeus; also compare Zenobia).
Zenobia
fem. proper name, from Greek Zenobia, literally "the force of Zeus," from Zen, collateral form of Zeus, + bia "strength, force," cognate with Sanskrit jya "force, power" (see Jain).
Zenonian (adj.)
1843, pertaining to one of two Greek thinkers: Zeno of Elea ("Zeno of the Paradoxes," 5c. B.C.E.), who disproved the possibility of motion; and Zeno of Citium (c. 300 B.C.E.), founder of stoicism.
zep (n.)
abbreviation of zeppelin, attested by 1915.
Zephaniah
masc. proper name, Biblical ninth of the prophets, from Hebrew Tzephanyah "the Lord has hidden."
zephyr (n.)
mid-14c., from Old English Zefferus, from Latin Zephyrus (source also of French zéphire, Spanish zefiro, Italian zeffiro), from Greek Zephyros "the west wind" (sometimes personified as a god), probably related to zophos "the west, the dark region, darkness, gloom." Extended sense of "mild breeze" is c.1600. Related: Zephyrean.
zeppelin (n.)
1900, from German Zeppelin, short for Zeppelinschiff "Zeppelin ship," after Count Ferdinand von Zeppelin (1838-1917), German general who perfected its design. Compare blimp.
zero (n.)
"figure which stands for naught in the Arabic notation," also "the absence of all quantity considered as quantity," c.1600, from French zéro or directly from Italian zero, from Medieval Latin zephirum, from Arabic sifr "cipher," translation of Sanskrit sunya-m "empty place, desert, naught" (see cipher (n.)).

A brief history of the invention of "zero" can be found here. Meaning "worthless person" is recorded from 1813. As an adjective from 1810. Zero tolerance first recorded 1972, originally U.S. political language. Zero-sum in game theory is from 1944 (von Neumann), indicating that if one player wins X amount the other or others must lose X amount.
zero (v.)
in zero in, 1944, from zero (n.); the image is from instrument adjustment to a setting of "zero" (1909 in this sense, originally in rifle-shooting). Related: Zeroed; zeroing.
zest (n.)
1670s, from French zeste "piece of orange or lemon peel used as a flavoring," of unknown origin. Sense of "thing that adds flavor" is 1709; that of "keen enjoyment" first attested 1791.
zestful (adj.)
1797, from zest + -ful. Related: Zestfully; zestfulness.
zesty (adj.)
1789, from zest + -y (2). Related: Zestily; zestiness.
zeta (n.)
sixth letter of the Greek alphabet; see zed.
zetetic (adj.)
"proceeding by inquiry," 1640s, from Modern Latin zeteticus, from Greek zetetikos "searching, inquiring," from zetetos, verbal adjective of zetein "seek for, inquire into." Related: Zetetical.
zeugma (n.)
1580s, "a single word (usually a verb or adjective) made to refer to two or more nouns in a sentence" (but properly applying to only one of them), from Greek zeugma, "a zeugma; that which is used for joining; boat bridge," literally "a yoking," from zeugnynai "to yoke" (see jugular).
Zeus
supreme god of the ancient Greeks and master of the others, 1706, from Greek, from PIE *dewos- "god" (cognates: Latin deus "god," Old Persian daiva- "demon, evil god," Old Church Slavonic deivai, Sanskrit deva-), from root *dyeu- "to gleam, to shine;" also the root of words for "sky" and "day" (see diurnal). The god-sense is originally "shining," but "whether as originally sun-god or as lightener" is not now clear.