The usual Old English word for cloud was weolcan. In Middle English, skie also originally meant cloud. The four fundamental types of cloud classification (cirrus, cumulus, stratus, nimbus) were proposed by British amateur meteorologist Luke Howard (1772-1864) in 1802. Figuratively, as something that casts a shadow, from early 15c.; hence under a cloud (c.1500).
Thanks for the A2A. Okay, the most famous song in the world is no doubt 'Happy Birthday', followed by various Christmas carols (since the majority of human population is Christian indeed and since Santa is a really famous guy), folk songs, army marches and national anthems.
Die Lorelei is arguably the most famous of German folk songs, certainly it is the best known in the English speaking world. The Lorelei (or Loreley) is a rock on the River Rhine, and herein it becomes a mermaid or siren, doing what these fascinating but deadly creatures do best, luring hapless sailors to their deaths.