Etymology
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Apollyon 
destroying angel of the bottomless pit in Rev. ix.11 (a name also sometimes given to the Devil), late 14c., from present participle of Greek apollyein "to destroy utterly" (from apo "from, away from" (see apo-) + olluein "to destroy"); a translation of Hebrew Abaddon (q.v.).
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Abaddon 

late 14c., used in Revelation ix.11 of "the angel of the bottomless pit," and by Milton of the pit itself, from Hebrew Abhaddon, literally "destruction," from abhadh "he perished." The Greek form was Apollyon.

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*apo- 
also *ap-, Proto-Indo-European root meaning "off, away."

It forms all or part of: ab-; abaft; ablaut; aft; after; apanthropy; aperitif; aperture; apo-; apocalypse; apocryphal; Apollyon; apology; apoplexy; apostle; apostrophe; apothecary; apotheosis; awk; awkward; ebb; eftsoons; of; off; offal; overt.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit apa "away from," Avestan apa "away from," Greek apo "from, away from; after; in descent from," Latin ab "away from, from," Gothic af, Old English of "away from."
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