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.