The English used to hold the Dutch in very low regard, on account of the hostilities between their two countries in the 17th century. As a consequence of the previous antagonism, there are numerous English phrases which portray Dutch items in a poor light:
Dutch comfort = cold comfort
Dutch concert = pandemonium
Dutch courage = the courage of drink
Dutch crossing = crossing the street slant wise
Dutch treat = each pays for their own expenses
Going Dutch = sharing the bill
The earliest example of 'double Dutch' that I have found is in John Davis' Travels of Four Years and a Half in the United States of America, 1803, in which the author spoke to a colleague in Welsh:
"Mr Adams - What devil language is that? Is it double Dutch coiled against the sun?"
That 'coiled up against the sun' appendage is now defunct, but appears frequently in early citations of the phrase.
An earlier reference to the difficulty in the understanding of Dutch by English speakers comes from Dibdin's Poor Jack, 1789:
"Why 'twas just all as one as High Dutch."
In recent years the computing term 'reverse Polish' has come to be synonymous with incomprehensibility. In fact 'reverse Polish' is a structured notation of formulae to enable machine calculations and is not open to interpretation or ambiguity. Nevertheless, many people find it confusing.
Double-Dutch is also the name of a children's skipping game, in which two ropes are used. This is referred to in Alice Gomme's The Traditional Games of England, Scotland, and Ireland,
1894. The game is still played, both on the street and at tournament level, most commonly in the USA, where skipping has the more descriptive name of 'jumping-rope'.