c.1220, "a cloud," from O.N.
sky "cloud," from P.Gmc.
*skeujam "cloud, cloud cover" (cf. O.E.
sceo, O.S.
scio "cloud;" O.H.G.
scuwo, O.E.
scua, O.N.
skuggi "shadow;" Goth.
skuggwa "mirror"), from PIE base
*(s)keu- "to cover, conceal" (see
hide (n.1)). Meaning "upper regions of the air" is attested from c.1300; replaced native
heofon in this sense (see
heaven). In M.E., the word can still mean both "cloud" and "heaven," as still in
the skies, originally "the clouds."
Sky-high is from 1818; phrase
the sky's the limit is attested from 1920.
Sky-dive first recorded 1965;
sky-writing is from 1923.