An Etymological Dictionary of the German Language/Sund
Sund, m., ‘sound, strait,’ early ModHG. only, a MidG. and LG. word; comp. AS. sund, E. sound, OIc. sund, ‘sea, strait.’ The connection with Goth. sundrô, ‘separated’ (see sonders), is open to objection on account of the meaning (Sund, lit. ‘division between countries and islands’?). It is preferable to link it with AS. and OIc. sund, n., ‘swimming,’ which is an abstract of schwimmen (sunda- for swm-tó-, allied to the root swem); by this assumption Sund is regarded as ‘the place where one can swim.’