"an overflowing, a flood," early 15c., from Latin inundationem (nominative inundatio) "an overflowing," noun of action from past-participle stem of inundare "to overflow," from in- "onto" (from PIE root *en "in") + undare "to flow," from unda "a wave," from PIE *unda-, nasalized form of root *wed- (1) "water; wet."
