The word sky came into English fairly early in the 13th century from the Old Norse word skiuja meaning a cloud. Skiuja (also related to the Proto-Germanic skeujam, meaning cloud or cloud cover) was related to the Indo-European bases skuggi (meaning shadow), skuggwa (meaning mirror) and (s)keu- meaning to cover or conceal. Other related cloud words were the Old English sceo and Old Saxon scio, both meaning cloud; Old High German scuwo, and Old English scua. Clearly when early English speakers and Vikings (both nautical cultures) looked at their skies they were seeing clouds!
Photo by vsharriman, used with permission under a Creative Commons 3.0 license.