For such a quotidian occurrence, cloud has a controversial and interesting etymology. Where it stems from is also the source of much confusion for etymologists, but many agree that it comes Proto-Indo-European as gleu, or "clay". This root, if it is correct, is also shared by "clay" and "glue". As PIE dropped a few letters, the word in Proto-Germanic then gained some. letters. It became klutaz, or "lump" because of clay's tendency to lump. This underwent even more inexplicable transitions and turned into clud, or the Old English word for "mass of rock". In Middle English this became cloud, but still meaning "a large formation of stone". The first use of cloud with its current meaning was around 1200 CE, when someone decided that those big airy lumps in the sky resembled those big dirty lumps on the ground and decided to adopt cloud for the thing in the sky, replacing the previously used weolcan and skie. The word "cloud" for rock evolved to be "clod" today.
Adam Aleksic, a leading contender for valedictorian of his high school, is a 208-month-old boy with disturbing interests in etymology, vexillology, geography, and law. Adam would like to one day visit Tajikistan and just won an essay contest on linguistics!
The Etymology Nerd