The term double Dutch means unintelligible speech.
It is likely that:
– under the influence of the traditional negative connotations attached to Dutch (cf. Dutch courage and Dutch uncle),
– and particularly because High Dutch has been used since the early 17th century in the sense of gibberish (read below),
in double Dutch:
– the word Dutch was chosen because it denotes a language that few people can speak (like Greek in it’s Greek to me),
– and double was chosen as a mere intensifier of the notion of unintelligibleness.
The earliest instance of double Dutch, unambiguously used in the current sense, that I have found is from The Bradford Observer (Bradford, Yorkshire) of Thursday 19th December 1839:
There is a valley near Kentmere, in Westmoreland, where, it is stated, the original language of the Danish inhabitants is still retained in so high a degree of purity, that a native of Denmark, at the present time, is able to hold a ready conversation with the peasantry in his own language.—Manchester Chronicle. [We apprehend that the paper from which we extract the above is in error, and that the language is not the Danish, but the real original “double Dutch,” in which Mr. Bellenden Ker’s Nursery Rhymes were originally written.]—Weekly Chronicle.
This article refers to John Bellenden Ker (John Bellenden Gawler – 1764-1842), English botanist, who developed a ludicrous theory in An Essay on the Archaiology [sic] of Popular English Phrases and Nursery Rhymes (Southampton and London, 1834); he explained in the preface, about “the frequent recurrence of phrases bearing a traditionary sense at variance with the terms in which they are vested”, such as “he has put the other’s nose out of joint”:
If we believe (which I do) the Anglo-Saxon and the Low-Saxon (still surviving, in the main, in what we now call the Dutch) were the same language, our own must at one period have been as these once were, also the same language.
It is to that period of our tongue, I have endeavoured to retrace the original form of the words which I believe then to have duly conveyed the sense of the phrases of the above category. By applying the sound of the words which constitute the modern phrase to others which it fitted in the Low-Saxon stage of our language, I have always found a sense, corresponding with that conveyed by the form under which they are now disguised, to be the result of the experiment. The following pages contain the proofs of this test.
In his book therefore, John Bellenden Ker ‘translated back’ into English phrases and nursery rhymes from the hypothetical early form of Dutch in which, according to him, they originally appeared.
In the specific sense of unintelligible speech, High Dutch is a loan translation from French haut allemand, meaning High German—cf. Dutch treat and footnote for the use of Dutch in the sense of German.
In this sense of gibberish, High Dutch is first recorded in The Iesuites catechisme. Or Examination of their doctrine (London, 1602), a translation by William Watson (circa 1559-1603), Roman Catholic priest and conspirator, of Le Catechisme des Iesuites : ou Examen de leur doctrine (Villefranche, 1602), by Étienne Pasquier (1529-1615), French author and lawyer:
I beseech you decyfer your doctrine that I may vnderstand it, for to say truth, this is high Dutch to me.
Ie vous prie me dechifrer ceste vostre doctrine : car pour vous dire le vray, ie n’y enten que le haut Alemand.
Likewise, Henry Baker (1698-1774), English natural philosopher and teacher of deaf people, and James Miller (1704-44), English playwright, poet, librettist and clergyman, used High Dutch to render haut allemand in The Amorous Quarrel, their translation of Le Dépit amoureux, by the French playwright Molière (Jean-Baptiste Poquelin – 1622-73):
(as published by Henry Baker and James Miller in The Works of Molière (volume 1 – London, 1739), with facing English and French texts)
My Father, tho’ he had one of the best Heads, never taught me any thing but my Mass-Book, which, though I have said it daily for fifty Years, is still High Dutch to me.
Mon pére, quoiqu’il eut la tête des meilleures,
Ne m’a jamais rien fait apprendre que mes heures,
Qui, depuis cinquante ans dites journellement,
Ne sont encor pour moi que du haut allemand.
Interestingly, Charles Heron Wall (1836-1905), English translator and schoolmaster, used double Dutch to render haut allemand in Lovers’ Quarrel, his translation of Le Dépit amoureux published in The Dramatic Works of Molière (volume 1 – London, 1876); in this translation, the above-mentioned passage is as follows:
My father was a very clear-headed man, but he never taught me anything but my prayers, and though I have said them daily now these fifty years, they are still double Dutch to me.
Note: Dutch has long been used in the general sense of German, including the language of the Netherlands as part of the Low-Dutch, i.e. Low-German, domain; for example, the following is from The Origin of the English Nation, by Edward A. Freeman, published in Macmillan’s Magazine (London) in March 1870:
We may […] divide the Teutonic languages into two classes, the High-Dutch and the Low. The former is the tongue of Southern or Upper Germany, the high lands away from the sea and near the sources of the rivers. The latter is the tongue of Northern, Lower, or Nether Germany, the lands near the sea and at the mouths of the rivers, the speech of what we specially call the Netherlands or Low Countries, and of the great plain stretching away eastward till we get out of the reach of Teutonic and Aryan languages altogether.
However, in Dutch auction, Dutch does refer specifically to the Netherlands.