'Sky' is Old Norse for 'cloud' — Vikings gave English the sky, which broadened from cloud to atmosphere.
The region of the atmosphere and outer space seen from the earth; the expanse above the horizon.
From Old Norse ský (cloud, overcast sky), from Proto-Germanic *skiwją (cloud, covering), from PIE *skew- or *skeud- (to cover, to conceal, to obscure). The PIE root *skew- also gave Latin obscūrus (covered over, dark) via *ob-skew-ro-, yielding English obscure. Old Norse ský entered English during the Scandinavian settlements of northern and eastern England (Danelaw period, 9th–11th centuries). The native Old English word for the heavens was heofon (heaven), which survives