mid-14c., overwhelmen, "to turn upside down, overthrow, knock over," from over- + Middle English whelmen "to turn upside down" (see whelm). Meaning "to submerge completely" is early 15c. Perhaps the connecting notion is a boat, etc., washed over, and overset, by a big wave. Figurative sense of "to bring to ruin" is attested from 1520s. Related: Overwhelmed; overwhelming; overwhelmingly.
mid-15c., "completely submerged or swamped," past-participle adjective from overwhelm. Figurative use by 1520s. Related: Overwhelmedness.
1590s, "to pour over, overwhelm in a flood, inundate;" see deluge (n.). Figurative sense of "overrun like a flood, pour over in overwhelming numbers" is from 1650s. Related: Deluged; deluging.
late 14c., "render (a region or place) lonely by depopulation or devastation; lay waste, ruin," from desolate (adj.) or Latin desolatus. Meaning "overwhelm with grief, make sorry or weary by affliction" is from 1520s. Related: Desolated; desolating.