What I wrote was actually somewhat speculative. More recent Dutch etymology research ( http://www.etymologiebank.nl/trefwoord/hangmat ) says that there is an early German find Hengmatten [1627; Pfeifer], more than 40 years before the first Dutch find (1669). Still, Philippa et al. claim, the German must be a loan translation from Dutch, both because it is a sailor's term in the Dutch Golden Age, and because a direct German derivation seems less plausible as it would have more sound changes.