An Etymological Dictionary of the German Language/Tümpel
Tümpel, m., ‘pool, deep part of a lake’; a MidG. form for MidHG. tümpfel, ‘deep place in flowing or standing water, pool, puddle,’ OHG. tumfilo, m., ‘whirlpool’ (whence the equiv. Ital. tonfano). A derivative of the nasalized root dup, ‘to be deep’ (see taufen, tief), which appears also in E. dimple, Du. dompelen, ‘to dive, plunge,’ dompelaar, ‘diver’ (bird). Pre-Teut. dhumb, ‘to be deep,’ is also assumed by Lith. dumbu, ‘to get hollow.’ Comp. also Topf.