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.
Entries linking to indoor
outdoor adj.
"that is done or used in the open air, not in the house," 1748, from out- + door. Out-of-door is from c. 1800.