Indoor

google

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early 18th century (superseding earlier within-door ): from in (as a preposition) + door.

wiktionary

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From (with)in +‎ door, first attested 1711. [1] [2]

etymonline

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indoor (adj.)

also in-door, 1670s, opposed to outdoor, contracted from within door; the form indoors is attested from 1759 (within-doors is from 1750); as an adverb from 1801.