the.com/cumulonimbus

means A towering, anvil-topped storm cloud that produces heavy rain, lightning, hail, and sometimes tornadoes.

from Latin all the way down: cumulus means 'heap' or 'pile' (the same word gives us a 'cumulative' stack), and nimbus means 'rain cloud' or 'dark cloud' — also the root of the English 'nimbus' for a glowing halo. Stack them together and you get, literally, a 'heaped-up rain cloud,' which is exactly what it is: ordinary puffy cumulus that kept absorbing warm, moist air and climbing until it became a tens-of-thousands-of-feet storm engine. The name was formalized as part of the international cloud-classification system that meteorologists built in the 19th century to standardize cloud-watching into actual science.