Often short for Microsoft Windows.
However, long before Microsoft decided to name their product this name, people had objects made of glass in the sides of their homes known as windows. Then there was this little open source project called the X windows system. And there probably were others who used the term as well.
Apparently, though, builders and perhaps MIT itself failed to trademark the word, and now the general population has their own take on what the word means.
Too bad Hormel doesn't own the trademark on windows - they might sue over trademark infringement...
RS: Other such misappropriated words include word, outlook, access, paint, project, ...