"in the open air, out of the house," 1817, from outdoor + adverbial genitive -s. As a noun, "open spaces," it is recorded from 1857; the great outdoors is recorded by 1932. Out a doors (adv.) is from 1520s.
he wanted to get outdoors a little
outcry
outdated
outdistance
outdo
outdoor
outdoors
outdoorsman
outen
outer
outermost
outerwear