[Middle English whelmen, to overturn, probably alteration (influenced by helmen, to cover) of whelven, from Old English -hwelfan (as in āhwelfan, to cover over).]
c.1300, probably from a parallel form of O.E. -hwielfan (W.Saxon), -hwelfan (Mercian), in ahwelfan "cover over;" probably altered by association with O.E. helmian "to cover" (see helmet).