From Latin 'cumulus' (a heap) — the same word later named the puffy cloud formation in 1803.
To gather together or acquire an increasing quantity of something over time; to pile up.
From Latin 'accumulare' (to heap up, pile up, amass), a prefixed form of 'cumulare' (to heap up, to heap together), from 'cumulus' (a heap, a pile, a mass), from PIE *kewH- or *ḱewH- (to swell, to be strong, to increase in mass). The PIE root *kewH- produced a family of swelling and heaping words: Latin 'cavus' (hollow — the opposite swelling inward), Latin 'tumor' (swelling), Greek 'kyma' (wave — a swelling of water), Greek 'kyrios' (powerful, the swelled-up one → 'Lord' in Christian Greek), and possibly English 'heap' itself. In Latin, 'cumulus' gave both the