Double Dutch: - Very interesting analysis here:
http://www.phrases.org.uk/meanings/double-dutch.html
I was particularly struck with:
"The Anglo-Dutch wars of the 17th and 18th centuries were acrimonious even by the usual standards of war. Following the conflicts the English came to hold the Dutch in very low regard and as a consequence there are numerous English phrases which portray them in an unflattering light, often as skinflints or drunkards. The common strand in all of these disparaging 'Dutch' expressions is that anything Dutch is the opposite of what it ought to be. Examples of these expressions are:
Dutch bargain - a bargain made when one is debilitated by drink - first recorded in 1654.
Dutch defence - a legal defence in which the defendant seeks clemency by deceitfully betraying others - 1749.
Dutch comfort - cold comfort; only good because things could have been worse - 1796.
Dutch metal/Dutch gold - a cheap alloy resembling gold - 1825.
Dutch courage - brash bravery induced by drink - 1826.
Dutch treat - no treat as such; each person pays for their own expenses - 1887.
Added to that list is 'double Dutch'. The Anglo-Dutch wars were a very long time ago and we are all friends now, but at this point we can introduce another reason for the English to have held on so long to hostile stereotyping of the Dutch, that is, the link with the UK's 20th century military rivals, the Germans. 'Dutch' was originally the generic name for both Germans and, as they were formally called, Hollanders. High Dutch was the language of southern Germany and Low Dutch the language of The Netherlands. "