apocalypse

First recorded in 1125–75; Middle English, from Late Latin apocalypsis, from Greek apokálypsis “revelation,” equivalent to apokalýp(tein) “to uncover, reveal” (apo- “away, off, apart” + kalýptein “to cover, conceal”) + -sis noun suffix; see apo-, -sis

Apocalypse comes from Greek apokálypsis “uncovering,” a derivative of the verb apokalýptein “to take the cover off,” a compound whose first element is the preposition and prefix apó, apo- “off, away.” The preposition apó has the same Proto-Indo-European origin as Latin ab, Sanskrit ápa, Gothic af (English off and of ), all meaning “away, away from.”
The second part of apokalýptein is the simple verb kalýptein “to cover, hide.” Kalýptein may be a derivative of the Proto-Indo-European root kel-, kol-, kəl- (and other variants). If so, kalýptein comes from the variant kəl-; the variant kēl- also forms Latin cēlāre “to hide.” The variant root kol- becomes hal- in Germanic, and the derivative noun haljō, literally “hidden place,” is the source of Old Norse Hel, goddess of the underworld, and English hell (the bad place).
The earliest recorded meaning of apocalypse (in Old English) was in reference to the name of the last book in the New Testament, also called Revelation. It recounts several prophetic visions of upheaval and destruction culminating in the Second Coming of Christ. In Middle English, apocalypse was extended to mean “any revelation or disclosure.” However, the meaning “any disaster or cataclysm” was not recorded until the late 19th century.

C13: from Late Latin apocalypsis, from Greek apokalupsis, from apokaluptein to disclose, from apo- + kaluptein to hide