Origins
The word 'sky' is one of the most striking examples of how thoroughly Old Norse reshaped the English language during the Viking Age. Before Scandinavian settlers arrived in Britain in the eighth through eleventh centuries, English speakers had no word 'sky.' They used 'heofon' (heaven) for both the physical expanse above their heads and the divine abode of God. The Norse loanword would eventually split this concept in two, giving English a distinction that most Germanic languages lack.
Old Norse 'ský' meant 'cloud,' not 'sky' in the modern English sense. It descended from Proto-Germanic *skiwją, itself from the PIE root *(s)kewH-, meaning 'to cover' or 'to conceal.' The same root produced Latin 'obscurus' (dark, hidden — source of English 'obscure'), showing how a single PIE concept of concealment branched into 'cloud' in Germanic and 'darkness' in Italic.
The word entered English through the everyday contact between Norse-speaking settlers in the Danelaw and their English-speaking neighbors. It first appears in English texts around 1200, initially retaining its Old Norse meaning of 'cloud.' The Ormulum, a Middle English biblical paraphrase from the late twelfth century, uses 'sky' to mean 'cloud,' and this sense persisted well into the fourteenth century.
Middle English
The semantic expansion from 'cloud' to 'the region where clouds are' to 'the entire visible expanse above' occurred gradually during the thirteenth and fourteenth centuries. By Chaucer's time, 'sky' could mean the whole dome of the atmosphere, though 'heaven' continued to be used in the same physical sense. The full displacement — where 'sky' became the default word for the physical atmosphere and 'heaven' was restricted primarily to religious contexts — was not complete until the Early Modern period.
This specialization created a distinction that is unusual among Germanic languages. German uses 'Himmel' for both 'sky' and 'heaven.' Dutch uses 'hemel' for both. Swedish and Danish do have 'sky,' but it has retained its original meaning of 'cloud' (Swedish 'himmel' serves as 'sky' and 'heaven,' while 'sky' means only 'cloud'). Only English pushed the Norse loanword to cover the entire atmosphere while simultaneously narrowing the native word to theological use.
The phonological development is straightforward. Old Norse 'ský' had a long /yː/ vowel (a rounded front vowel). In English, this was adapted as /iː/ and eventually diphthongized to /aɪ/ during the Great Vowel Shift, producing the modern pronunciation. The initial /sk-/ cluster is itself a marker of Norse origin in English — native Old English words with initial /sk-/ had already shifted to /ʃ/ (written 'sh'), so pairs like 'shirt' (English) versus 'skirt' (Norse) and 'ship' (English) versus 'skip' (Norse) reveal their different origins through this sound alone.
Word Formation
The word generated a rich family of compounds in English. 'Skyline' appeared in the seventeenth century, 'skylight' in the same period, and 'skyscraper' — originally a nautical term for a tall sail — was transferred to buildings in the 1880s during the first wave of steel-framed construction in Chicago. 'Skylark,' combining Norse 'sky' with native English 'lark,' names a bird whose most distinctive behavior is singing while ascending into the sky.