Old English flōd "a flowing of water, tide, an overflowing of land by water, a deluge, Noah's Flood; mass of water, river, sea, wave," from Proto-Germanic *floduz "flowing water, deluge" (source also of Old Frisian flod, Old Norse floð, Middle Dutch vloet, Dutch vloed, German Flut, Gothic flodus), from suffixed form of PIE verbal root *pleu- "to flow" (also the source of flow). In early modern English often floud. Figurative use, "a great quantity, a sudden abundance," by mid-14c.
1660s, "to overflow" (transitive), from flood (n.). Intransitive sense "to rise in a flood" is from 1755. Related: Flooded; flooding.