Dutch (adj.) Look up Dutch at Dictionary.com
late 14c., used at first of Germans generally, after c. 1600 in the narrower sense "Hollanders;" from Middle Dutch duutsch, from Old High German duitisc, from Proto-Germanic *theudo "popular, national" (source of Modern German Deutsch; see Teutonic). It corresponds to the Old English adjective þeodisc "belonging to the people," which was used especially of the common language of Germanic people, a derivative of the Old English noun þeod "people, race, nation." From the same PIE root (*teuta- "people") come Old Irish tuoth "people," Old Lithuanian tauta "people," Old Prussian tauto "country," Oscan touto "community."

As a language name, it is first attested as Latin theodice (786 C.E.) in correspondence between Charlemagne's court and the Pope, in reference to a synodical conference in Mercia; thus it refers to Old English. First use in reference to the German language (as opposed to a Germanic one) is two years later. The sense was extended from the language to the people who spoke it (in German, Diutisklant, ancestor of Deutschland, was in use by 13c.).

Sense in England narrowed to "of the Netherlands" in 17c., after they became a united, independent state and the focus of English attention and rivalry. In Holland, Duits (formerly duitsch) is used of the people of Germany. The old use of Dutch for "German" continued in America (Irving and Cooper still distinguish High Dutch "German" and Low Dutch "Dutch") and survives in Pennsylvania Dutch for the descendants of religious sects that immigrated from the Rhineland and Switzerland and their language.

Since c. 1600, Dutch (adj.) has been a "pejorative label pinned by English speakers on almost anything they regard as inferior, irregular, or contrary to 'normal' (i.e., their own) practice" [Rawson]. E.g. Dutch treat (1887), Dutch uncle (1838), etc. -- probably exceeded in such usage only by Indian and Irish -- reflecting first British commercial and military rivalry and later heavy German immigration to U.S.
The Dutch themselves spoke English well enough to understand the unsavory connotations of the label and in 1934 Dutch officials were ordered by their government to stop using the term Dutch. Instead, they were to rewrite their sentences so as to employ the official The Netherlands. [Rawson]
Dutch oven is from 1769; OED lists it among the words describing things from Holland, but perhaps it is here used in the slighting sense. Dutch elm disease (1927) so called because it was first discovered in Holland (caused by fungus Ceratocystis ulmi).