Plattdeutsch

late 14c., of language, "German, non-Scandinavian continental Germanic," also as a noun, "a German language;" also in Duche-lond "Germany." By mid-15c. distinguished into Higher and Lower, and used after c. 1600 in the narrower sense "Hollanders, residents of the Netherlands." From Middle Dutch duutsch, from Old High German duitisc, from Proto-Germanic *theudō "popular, national" (source of Modern German Deutsch), from PIE *teuta- "tribe" (compare Teutonic).

It corresponds to the Old English adjective þeodisc "belonging to the people," which was used especially of the common language of Germanic people (as opposed to Latin), a derivative of the Old English noun þeod "people, race, nation." The language name is first attested in Latin as 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. Its first use in reference to a 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.).

The sense in of the adjective in English 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. ironical Dutch treat, of each person paying for himself (1887), Dutch courage "boldness inspired by intoxicating spirits" (1809), nautical Dutch talent "any piece of work not done in shipshape style (1867), 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.

Dutch concert, a concert in which each one sings his own song at the same time that his neighbor sings his; or a concert in which each one sings a verse of any song he pleases, some well-known chorus being sung after each verse. [Century Dictionary, 1897]

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). A Dutch uncle (1838) is one who is kindly severe and direct. 

"a native of Germany," 1520s, from Latin Germanus (adjective and noun, plural Germani), first attested in writings of Julius Caesar, who used Germani to designate a group of tribes in northeastern Gaul, of unknown origin and considered to be neither Latin nor Germanic. Perhaps originally the name of an individual tribe, but Gaulish (Celtic) origins have been proposed, from words perhaps originally meaning "noisy" (compare Old Irish garim "to shout") or "neighbor" (compare Old Irish gair "neighbor"). Middle English had Germayns (plural, late 14c.), but only in the sense "ancient Teuton, member of the Germanic tribes." The earlier English word was Almain (early 14c., via French; see Alemanni) or Dutch. Shakespeare and Marlowe have Almain for "German; a German."

Þe empere passede from þe Grees to þe Frenschemen and to þe Germans, þat beeþ Almayns. [Ranulph Higden’s "Polychronicon," mid-14c., John Trevisa's translation,  1380s]

Their name for themselves, die Deutschen (see Dutch), dates from 12c. Roman writers also used Teutoni as a German tribal name, and writers in Latin after about 875 commonly refer to the German language as teutonicus (see Teutonic). Meaning "the German language" in English is from 1748. High German (1823 in English) and Low German as a division of dialects is geographical: High German (from 16c. established as the literary language) was the German spoken in the upland regions in southern Germany, Low German (often including Dutch, Frisian, Flemish), also called Plattdeutsch was spoken in the regions near the North Sea. In the U.S. German also was used of descendants of settlers from Germany.