Origins
The word 'cloud' has undergone one of the more dramatic semantic shifts in the history of English. Old English 'clūd' meant 'mass of rock,' 'hill,' or 'rocky eminence' — it had nothing to do with the sky. The word descends from Proto-Germanic *klūtaz (lump, mass, block), and its original sense survives in the closely related word 'clod' (a lump of earth) and possibly 'clot' (a lump of coagulated blood).
The transfer from 'rock mass' to 'vapor mass' occurred during the Middle English period, roughly in the 13th century. The metaphor is visual: large cumulus clouds resemble enormous, billowing, lumpy formations — not unlike the rocky masses and hillocks that 'clūd' originally denoted. Once the atmospheric meaning took hold, it rapidly displaced the geological one, and by 1400 'cloud' meant exclusively the sky phenomenon. The original 'rock' sense survives only in specialized dialectal usage and in etymological dictionaries.
The Old English word for an atmospheric cloud was 'wolcen' (also spelled 'wolcn'), which could mean both 'cloud' and 'sky.' This word is cognate with Old High German 'wolkan' (modern German 'Wolke,' cloud), Old Norse 'ský' (cloud — which, confusingly, is the source of English 'sky'), and possibly Old Saxon 'wolkan.' The displacement of 'wolcen' by 'cloud' is one of many examples of native English words being replaced not by foreign borrowings but by other native words with shifted meanings.
Germanic Development
The Germanic cognates of 'cloud' in its original sense are Dutch 'kluit' (clod, lump of earth) and German 'Klotz' (block, log). These preserve the 'lump/mass' meaning that Old English 'clūd' originally carried. English 'clod' is a variant of the same word — 'cloud' and 'clod' are doublets from the same Proto-Germanic root, differentiated by slight phonological divergence.
Luke Howard's 1802 classification of clouds into Latin categories — cumulus (heap), stratus (layer), cirrus (curl), and nimbus (rain-cloud) — gave English the technical vocabulary for cloud types. But the everyday English word 'cloud' itself, with its hidden history as 'rock,' predates all scientific classification by centuries.
The figurative extensions of 'cloud' all postdate the atmospheric meaning. 'Under a cloud' (under suspicion, in disgrace) dates from the 1500s. 'Cloud nine' (a state of bliss) is mid-twentieth-century American English, possibly derived from a classification system in which cloud type nine (cumulonimbus) was the highest-reaching. 'To cloud' as a verb meaning 'to obscure' or 'to make gloomy' dates from the 1400s. 'Cloud computing,' the 21st-century term for remote data storage, draws on the older convention of using a cloud icon to represent the internet in network diagrams — the cloud meaning 'something vast and nebulous that you connect to without seeing its internal structure.'