early 14c., probably from a parallel form of Old English -hwielfan (West Saxon), -hwelfan (Mercian), in ahwelfan "cover over;" probably altered by association with Old English helmian "to cover," from Proto-Germanic *hwalbjan, from PIE *kuolp- "to bend, turn" (see gulf (n.)).
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.