Etymology
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occult (adj.)

1530s, "secret, not divulged," from French occulte and directly from Latin occultus "hidden, concealed, secret," past participle of occulere "cover over, conceal," from assimilated form of ob "over" (see ob-) + a verb related to celare "to hide" (from PIE root *kel- (1) "to cover, conceal, save"). Meaning "not apprehended by the mind, beyond the range of understanding" is from 1540s. The association with the supernatural sciences (magic, alchemy, astrology, etc.) dates from 1630s. A verb occult "to keep secret, conceal" (c.1500, from Latin occultare) is obsolete.

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occultation (n.)

early 15c., occultacioun, "disguise or concealment of identity," from Latin occultationem (nominative occultatio), noun of action from past-participle stem of occultare "to hide, conceal," frequentative of occulere "to cover over, conceal" (see occult).

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occultism (n.)

"the doctrine, principles, or practices of occult sciences," 1870, from occult + -ism. Related: Occultist.

Occultism, I should add, is to be distinguished from the primitive magic described by anthropologists, which is prescientific, prephilosophical, and perhaps prereligious, whereas occultism is a pseudo-science or system of pseudo-sciences, often supported by an irrationalist philosophy, and always exploiting the disintegrated débris of preexisting religions. [E.R. Dodds, "The Greeks and the Irrational," 1951, footnote]
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occupancy (n.)
1590s, "condition of being an occupant;" from occupant + abstract noun suffix -cy. Meaning "fact of occupying" is from 1833; that of "proportion of available space that is occupied" is attested by 1974.
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occupant (n.)

1590s, "one who takes possession of something having no owner," from French occupant (15c.) or directly from Latin occupantem (nominative occupans), present participle of occupare "to take possession of" (see occupy). Earlier noun form was ocupier (early 14c.).

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occupation (n.)

early 14c., "fact of holding or possessing;" mid-14c., "a being employed in something," also "a particular action," from Old French occupacion "pursuit, work, employment; occupancy, occupation" (12c.), from Latin occupationem (nominative occupatio) "a taking possession; business, employment," noun of action from past-participle stem of occupare (see occupy). Meaning "employment, business in which one engages" is late 14c. That of "condition of being held and ruled by troops of another country" is from 1940.

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occupational (adj.)

"of or pertaining to a particular occupation, calling, or trade," 1850, from occupation + -al (1). Occupational therapy is attested by 1918; occupational risk by 1951. Related: Occupationally.

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occupied (adj.)
late 15c., past-participle adjective from occupy (v.). Of countries overrun by others, from 1940, originally with reference to France.
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occupier (n.)

late 14c., occupiour, "on who takes or holds possession" (of lands, manors, a benefice, etc.), agent noun from occupy.

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occupy (v.)

mid-14c., occupien, "to take possession of and retain or keep," also "to take up space or room or time; employ (someone)," irregularly borrowed from Old French ocuper, occuper "occupy (a person or place), hold, seize" (13c.) or directly from Latin occupare "take over, seize, take into possession, possess, occupy," from ob "over" (see ob-) + intensive form of capere "to grasp, seize," from PIE root *kap- "to grasp."

The final syllable of the English word is difficult to explain, but it is as old as the record; perhaps it is from a modification made in Anglo-French. During 16c.-17c. the word was a common euphemism for "have sexual intercourse with" (a sense attested from early 15c.), which caused it to fall from polite usage.

"A captaine? Gods light these villaines wil make the word as odious as the word occupy, which was an excellent good worde before it was il sorted." [Doll Tearsheet in "2 Henry IV"]

During the same time occupant could mean "prostitute." Related: Occupied; occupying.

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