Etymology
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Words related to *apo-

ab- 
word-forming element meaning "away, from, from off, down," denoting disjunction, separation, departure; from Latin ab (prep.) "off, away from" in reference to space or distance, also of time, from PIE root *apo- "off, away" (also the source of Greek apo "off, away from, from," Sanskrit apa "away from," Gothic af, English of, off; see apo-).

The Latin word also denoted "agency by; source, origin; relation to, in consequence of." Since classical times usually reduced to a- before -m-, -p-, or -v-; typically abs- before -c-, -q-, or -t-.
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abaft (adv.)
"in or farther toward the back part (of a ship)," as opposed to forward, 1590s, from Middle English on baft (late 13c.) "back, behind, to the rear," from Old English on bæftan. For first element, see a- (1). The second component is itself a compound of be "by" (see by) and æftan "aft" (see aft). The word has been saved by the sailors (the stern being the "after" part of a vessel), the rest of the language having left it in Middle English.
ablaut (n.)

"systematic vowel alteration in the root of a word to indicate shades of meaning or tense," a characteristic of Indo-European languages, 1845, from German Ablaut, literally "off-sound" ("off" here denoting substitution), coined by J.P. Zweigel in 1568 from ab "off" (from Old High German aba "off, away from," from PIE root *apo- "off, away") + Laut "sound, tone" (from Old High German hlut, from Proto-Germanic *hludaz "heard, loud," from suffixed form of PIE root *kleu- "to hear"). The word was popularized by Jakob Grimm and Franz Bopp. The process is what makes strong verbs in Germanic. An example is bind/band/bond/bound + (German) Bund. Compare umlaut.

In our language, it seems to us that the uncouthness of such compounds as Upsound, Offsound, and Insound, could hardly be compensated by any advantage to be derived from their use; and we therefore purpose, in the course of this work, where any of these terms occur in the original, to retain them in their German shape. Of these terms, Ablaut and Umlaut are those which chiefly, if not alone, are used by our author. [from footnote in translation of Bopp's "Comparative Grammar," London, 1845]
aft (adv.)

Old English æftan "from behind, behind, farthest back," superlative of Old English æf, af, of "away, away from, off" (from PIE root *apo- "off, away"). Cognate with Old Frisian eft "later, afterwards; as well," Old Norse eft "after," Middle Dutch echter, efter "later, again," Gothic afta "behind, past." The Germanic superlative suffix *-ta corresponds to PIE *-to (compare Greek prōtos "first," superlative of pro "before"). The word is now purely nautical, "in, near, or toward the stern of a ship."

after (adv., prep.)

Old English æfter "behind; later in time" (adv.); "behind in place; later than in time; in pursuit, following with intent to overtake" (prep.), from of "off" (see off (adv.)) + -ter, a comparative suffix; thus the original meaning was "more away, farther off." Compare Old Norse eptir "after," Old Frisian efter, Dutch achter, Old High German aftar, Gothic aftra "behind;" also see aft. Cognate with Greek apotero "farther off," Old Persian apataram "further."

From c. 1300 as "in imitation of." As a conjunction, "subsequent to the time that," from late Old English. After hours "hours after regular working hours" is from 1814. Afterwit "wisdom that comes too late" is attested from c. 1500 but seems to have fallen from use. After you as an expression in yielding precedence is recorded by 1650.

apanthropy (n.)

"aversion to human company, love of solitude," 1753, nativized form of Greek apanthropia, abstract noun from apanthropos "unsocial," from assimilated form of apo "off, away from" (see apo-) + anthrōpos "man, human" (see anthropo-). Related: Apanthropic.

aperitif (n.)

"alcoholic drink taken before a meal to stimulate the appetite," 1890, from French apéritif "laxative, laxative liqueur," literally "opening," from Latin aperitivus, from aperire "to open, uncover," from PIE compound *ap-wer-yo- from *ap- "off, away" (see apo-) + root *wer- (4) "to cover." A doublet of apertive.

aperture (n.)

early 15c., "an opening, hole, orifice," from Latin apertura "an opening," from apertus, past participle of aperire "to open, uncover," from PIE compound *ap-wer-yo- from *ap- "off, away" (see apo-) + root *wer- (4) "to cover." In optics, diameter of the exposed part of a telescope, microscope, etc., 1660s.

apo- 
before vowels ap-, word-forming element meaning "of, from, away from; separate, apart from, free from," from Greek apo "from, away from; after; in descent from," in compounds, "asunder, off; finishing, completing; back again," of time, "after," of origin, "sprung from, descended from; because of," from PIE root *apo- "off, away" (source also of Sanskrit apa "away from," Avestan apa "away from," Latin ab "away from, from," Gothic af, Old English of "away from," Modern English of, off).
apocalypse (n.)
Origin and meaning of apocalypse

late 14c., "revelation, disclosure," from Church Latin apocalypsis "revelation," from Greek apokalyptein "uncover, disclose, reveal," from apo "off, away from" (see apo-) + kalyptein "to cover, conceal," from PIE root *kel- (1) "to cover, conceal, save." The Christian end-of-the-world story is part of the revelation in John of Patmos' book "Apokalypsis" (a title rendered into English as pocalipsis c. 1050, "Apocalypse" c. 1230, and "Revelation" by Wyclif c. 1380).

Its general sense in Middle English was "insight, vision; hallucination." The meaning "a cataclysmic event" is modern (not in OED 2nd ed., 1989); apocalypticism "belief in an imminent end of the present world" is from 1858. As agent nouns, "author or interpreter of the 'Apocalypse,'" apocalypst (1829), apocalypt (1834), and apocalyptist (1824) have been tried.