c.1200, "a cloud," from Old Norse sky "cloud," from Proto-Germanic *skeujam "cloud, cloud cover" (cf. Old English sceo, Old Saxon scio "cloud, region of the clouds, sky;" Old High German scuwo, Old English scua, Old Norse skuggi "shadow;" Gothic skuggwa "mirror"), from PIE root *(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 Middle English, the word can still mean both "cloud" and "heaven," as still in the skies, originally "the clouds." Sky-high is from 1812; phrase the sky's the limit is attested from 1908. Sky-dive first recorded 1965; sky-writing is from 1922.