cumulus

The word 'cumulus' is a direct borrowing from Latin, where cumulus meant a heap or pile. This Latin term is believed to be derived from the Proto-Indo-European root keu- meaning 'to swell' or 'to be strong', which also gave rise to words like 'cume' (obsolete for 'come') and is related to concepts of accumulation. The meteorological application, describing a cloud that appears heaped or piled up, was coined in the early 19th century by the English meteorologist Luke Howard as part of his pioneering cloud classification system. Thus, the literal ancient meaning of a physical heap evolved logically into the modern abstract term for a specific, puffy cloud formation.

Structure: cumul (heap) + us (noun suffix) cumul (from Latin cumulus, meaning 'heap', 'pile', 'mass'); us (a Latin noun suffix used to form masculine nouns of the second declension, often borrowed directly into English).