flood-plain (n.)

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.

"level country, expanse of level or nearly level ground," c. 1300 (in reference to Salisbury Plain), from Old French plain "open countryside," from Latin planum "level ground, plain," noun use of neuter of planus (adj.) "flat, even, level" (from PIE root *pele- (2) "flat; to spread"). Latin planum was used for "level ground" but much more common was campus.