Etymology
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vacancy (n.)
1570s, "a vacating;" c. 1600, "state of being vacant," from Late Latin vacantia, from Latin vacans "empty, unoccupied," present participle of vacare "be empty," from PIE *wak-, extended form of root *eue- "to leave, abandon, give out." From 1690s as "a vacant office or post;" meaning "available room at a hotel" is recorded from 1953. Related: Vacance (1530s); vacancies.
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vacuity (n.)
late 14c., "hollow space," from Latin vacuitas "empty space, emptiness, absence, vacancy, freedom," from vacuus "empty," from PIE *wak-, extended form of root *eue- "to leave, abandon, give out." Originally in anatomy. Meaning "vacancy of mind or thought" is attested from 1590s.
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cooptation (n.)

also co-optation, 1530s, "choice, selection, mutual choice, election to fill a vacancy" on a committee, board, or society, from Latin cooptationem (nominative cooptatio) "election," noun of action from past-participle stem of cooptare "to elect, to choose as a colleague or member of one's tribe," from assimilated form of com "with, together" (see com-) + optare "choose" (see option (n.)). Related: Cooptative.

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interregnum (n.)
1580s, from Latin interregnum "an interval between two reigns," literally "between-reign," from inter "between" (see inter-) + regnum "kingship, dominion, rule, realm," related to regere "to rule, to direct, keep straight, guide" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule"). In the republic, it meant a vacancy in the consulate. The earlier English noun was interreign (1530s), from French interrègne (14c.).
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vacation (n.)

late 14c., "freedom from obligations, leisure, release" (from some activity or occupation), from Old French vacacion "vacancy, vacant position" (14c.) and directly from Latin vacationem (nominative vacatio) "leisure, freedom, exemption, a being free from duty, immunity earned by service," noun of state from past-participle stem of vacare "be empty, free, or at leisure," from PIE *wak-, extended form of root *eue- "to leave, abandon, give out."

Meanings "state of being unoccupied," "process of vacating" in English are early 15c. Meaning "formal suspension of activity, time in which there is an intermission of usual employment" (in reference to schools, courts, etc.) is recorded from mid-15c. As the U.S. equivalent of what in Britain is called a holiday, it is attested from 1878.

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recruit (n.)

"military reinforcement, one of a newly raised body of soldiers or sailors to supply a military deficiency," 1640s, from recruit (v.), replacing earlier recrew, recrue; or from obsolete French recrute, alteration of recreue "a supply," recrue "a levy of troops" (late 16c.). This is a Picardy or Hainault dialect variant of recrue "a levy, a recruit," literally "a new growth," from Old French recreu (12c.), past participle of recreistre "grow or increase again," from re- "again" (see re-) + creistre "to grow," from Latin crescere "to grow" (from PIE root *ker- (2) "to grow").

"The French word first appeared in literary use in gazettes published in Holland, and was disapproved of by French writers in the latter part of the 17th c." [OED]. The French word also is the source of Dutch recruut, German Recrut, Swedish rekryt. The general sense of "one who has newly filled a vacancy in any body or class of persons" also is from 1640s.

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