Plain English Meaning
A tall, fast-growing puffy cloud that looks like a cauliflower shooting upward. It is not a thunderstorm yet, but it is heading that way and signals rough air and unstable conditions.
Full Definition
A cumulus cloud that has grown vertically into a tall, narrow column with strong upward development, but has not yet matured into a thunderstorm (cumulonimbus). In a METAR, TCU is reported in the remarks or sky condition section to indicate significant convective activity that may produce turbulence, icing, and rapidly worsening weather.
Context Anchor
Seen in the cloud section of a METAR, often attached to a reported cloud layer height.
Derivation
From Latin cumulus meaning a heap or pile. The word towering simply describes the cloud's tall, building shape. Together they a heaped cloud that is still piling upward.