Is your source really sure it's from germanus or is this a folk etymology? I'd wager a guess that it's German measles just because this is a colloquial way of indicating "not true/not standard/from elsewhere" -- like Welsh rabbit or French kissing or Dutch courage.A touch, a touch, I do confess!
Neither the Oxford English Dictionary nor Webster’s New World Dictionary gives the origin of the term "German measles," but both uppercase the G, and the OED goes further by listing the illness under "German" with a capital G, not "german" with a little g. So Beth’s argument is looking pretty good, and Ye Humble Blogsmith is feeling more humble than usual.
Can anyone point us to a definitive origin of "German measles," or is this one of the (countless) terms for which no such definitive origin is known?
As for whether "German measles" might have been a jingoistic way of saying "faux measles," it’s interesting that the earliest appearances of "German measles" cited by the OED are medical journals of the late 19th century, the Bismarck era, when anti-German sentiment may have been at high tide.
Still, the term isn’t as obviously insulting as "Welsh rabbit" or "Dutch courage" or "Irish taxi" (a wheelbarrow). And "French kiss" seems, to modern sensibilities, downright complimentary!