late 15c., "that which is heaped up, an accumulated mass," from Latin accumulationem (nominative accumulatio) "a heaping up," noun of action from past-participle stem of accumulare "to heap up, amass," from ad "to," here perhaps emphatic (see ad-), + cumulare "heap up," from cumulus "heap" (from suffixed form of PIE root *keue- "to swell"). Meaning "act of heaping up" is from c. 1600.
c. 1600, "formed by adding to, increasing inorganically" (a sense now obsolete), from Latin cumulatus, past participle of cumulare "to heap," from cumulus "heap" (from suffixed form of PIE root *keue- "to swell") + -ive. Meaning "increasing by successive additions" is from 1640s (implied in cumulatively).
1610s, "act of heaping together;" 1620s, "a heap, that which is piled up," noun of action from cumulate.
1650s, "a heap," from Latin cumulus "a heap, pile, mass, surplus," from PIE *ku-m-olo-, suffixed shortened form of root *keue- "to swell." Meteorological use for "rounded mass of clouds, snowy white at the top with a darker, horizontal base" is attested by 1803.