An Etymological Dictionary of the German Language/Sumpf
Sumpf, m., ‘swamp, bog, marsh,’ from the equiv. MidHG. sumpf (wanting in OHG., in which sumft is used). Corresponding to Du. somp, and with an old gradation E. swamp (dial. sump). OHG. giswumft and Goth. swumfel, ‘pond,’ are differently derived. Its connection with schwimmen (Sumpf, ‘porous soil’?) is very dubious; it is preferable to connect it with OIc. svǫppr, ‘sponge.’ The Teut. root was probably swemp; E. dial. swanky, ‘marshy,’ may point to an orig. swenq.