The word "thundercloud" is a transparent Modern English compound, formed by directly combining two native English words. The first element, "thunder," descends from an ancient onomatopoeic Proto-Indo-European root imitating a roaring sound, which evolved into the name for both the meteorological phenomenon and the Norse god associated with it. The second element, "cloud," underwent a remarkable semantic shift from its Old English meaning of "rock" or "hill" to its current meteorological sense, likely because dense cloud formations were visually analogous to distant hills. The compound's logic is perfectly literal, describing a specific type of cloud defined by its capacity to produce thunder, showcasing how English readily creates precise descriptive terms from its Germanic lexical stock.

Structure: thunder (thunder) + cloud (cloud) - thunder: From Old English þunor, meaning "thunder, thunderclap," and by extension, the god Thor. Its Proto-Indo-European root is (s)tenh₂-, meaning "to thunder, roar." In this compound, it functions as a noun modifier. - cloud: From Old English clūd, meaning "mass of rock, hill," later shifting to "mass of cloud" in the 13th century, influenced by its resemblance to a hill. Its origin is uncertain but may be from a Proto-Germanic root. In this compound, it functions as the head noun.