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.
*keuə-, Proto-Indo-European root meaning "to swell," also "vault, hole."
It forms all or part of: accumulate; accumulation; cave; cavern; cavity; coeliac; church; codeine; coelacanth; coeliac; coelomate; concave; cumulate; cumulative; cumulus; enceinte; excavate; kirk; kymatology; Kyrie eleison.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit svayati "swells up, is strong;" Greek kyein "to swell," koilos "hollow, hollowed out, spacious, deep;" Latin cumulus "a heap, pile, mass, surplus;" Lithuanian šaunas "firm, solid, fit, capable;" Middle Irish cua "hollow;" Armenian soyl "cavity."
early 15c., "accumulation of morbid matter in the body," from Old French congestion (14c.) and directly from Latin congestionem (nominative congestio) "a heaping up," noun of action from past participle stem of congerere "to bring together, pile up," from assimilated form of com "with, together" (see con-) + gerere "to carry, perform" (see gest). Medical sense "unnatural accumulation of fluid" is from 1630s; meaning "a crowding together of people, traffic, etc." is from 1883.
early 15c., "accumulated;" 1570s, "heaped up, gathered into a mass," past-participle adjective from congest. Medical sense of "containing an unnatural accumulation of fluid" is from 1758. Meaning "overcrowded" is recorded from 1862.
"morbid accumulation of watery liquid in a part of the body," late 13c., a shortening of Middle English ydropsy, idropsie, from Old French idropsie and directly from Latin hydropsis, from Greek hydrops (genitive hydropos) "dropsy," from hydor "water" (from suffixed form of PIE root *wed- (1) "water; wet").