Well I'll be damned. Turns out the English word "quack" comes from the Dutch "kwaksalver" -- seller of salve.
Isn't this the great thing about this forum - you're always learning something new? For anyone else interested the etymology from the OED is:
[a. early mod.Du. (16th c.) quacksalver (Kilian; mod.Du. kwakzalver), whence also G. quacksalber, Sw. qvacksalfvare: the second element is f. salf, zalf salve, ointment, and the first is commonly regarded as the stem of quacken (mod.Du. kwakken) to quack.
On this view a quacksalver is one who ‘quacks’ or boasts about the virtues of his salves; it has however been suggested that quack- or kwak- may mean ‘to work in a feeble bungling fashion’ (Franck).]