Theodiscus is a Medieval Latin term literally meaning "popular" or "of the people". In Medieval Western Europe non-native Latin was the language of science, church and administration, hence theodiscus was used as an antonym of Latin, to refer to the "native language spoken by the general populace". The term was subsequently used in the Frankish Empire to denote the native Germanic vernaculars. As such, it was no longer used as antonym of Latin, but of walhisk, a language descendent from Latin, but nevertheless the speech of the general populace as well. In doing so theodiscus effectively obtained the meaning of "Germanic", or more specifically one of its local varieties – resulting in the English exonym Dutch, the German endonym Deutsch, and the Dutch exonym Duits, all of which are all cognates of theodiscus.
Theodiscus is derived from West Germanic *þiudisk, from Proto-Germanic *þiudiskaz. The stem of this word, *þeudō, meant "people" in Proto-Germanic, and *-iskaz was an adjective-forming suffix, of which -ish is the Modern English cognate with the same meaning. The Proto-Indo-European word *tewtéh₂ ("tribe", "people"), which is commonly reconstructed as the basis of the word, is related to Lithuanian tautà ("nation"), Old Irish túath ("tribe", "people") and Oscan touto ("community").
The word existed in Old English as þēodisc ("speech", "public", 'native"), came into Middle English as thede ("nation", "people") and was extinct in Early Modern English, although surviving in the English place name Thetford, "public ford" and in the exonym Dutch. It survives as the Icelandic word þjóð for "people, nation", the Norwegian word tjod for "people", "nation", and the word "German" in many languages including German Deutsch, Dutch Duits, Yiddish דײַטש, Danish tysk, Norwegian tysk, Swedish tyska and Italian tedesco.
The word theodism, a neologism for a branch of Germanic neopaganism, is based on the Gothic form of the word, where þiudisko took on the meaning of "pagan". Proto-Slavic borrowed the word as *ťuďь with the meaning "foreign" (thus the opposite of the original meaning "native"), giving rise to modern Polish cudzy, Czech cizí, Serbo-Croat tuđi and for example Russian чужой.
While morphologically similar, the Latin root Teutonic for "Germanic" is more distantly related, and originally a name of a Celtic or Germanic tribe that inhabited coastal Germany. It came probably via Celtic from Proto-Germanic *þeudanaz ("ruler", "leader of the people"), from Proto-Indo-European *tewtéh₂ ("people", "tribe").
The first recorded use of "theodisce" as a reference to a Germanic language was Old English. Around the year 786 the Bishop of Ostia writes to Pope Adrian I about a synod taking place in Corbridge, England; the decisions were later read aloud elsewhere "tam Latine quam theodisce" meaning "in Latin as well as the vernacular". However, in Great Britain the term Englisc was already common from the Early Middle Ages onwards, with the Angles, Saxons and Jutes being collectively referred to as 'English' as early as 731.
The second recorded use of "theodisca" as a reference to a Germanic language was Old High German. In 788, the Annals of the Frankish Kingdom report the punishment of a Bavarian duke: "quod theodisca lingua herisliz dictum", meaning "known in the language of the people as herisliz". Herisliz is a German word now obsolete: the "slicing", i.e. tearing apart of the "Heer" (Desertion).
In German dialects, a large amount of forms of "theodiscus" existed throughout the Middle Ages and which all referred to either the broader Romance/Germanic dichotomy in the West and South or the Slavic/Germanic bipartition in the East. In Old High German both diutisk and diutisc are known, that developed in Middle High German as diutsc. In Middle Low German it was known as düdesch and Modern Low German as dütsch. However, in German, the use of the term referring to Germans specifically as opposed to people speaking Germanic languages in general evolves during the Early Modern Period and it is in the late 17th and 18th century that the modern meaning of Deutsch is established.
In the United States the German autonym Deutsch would sometimes be rendered as Dutch, due to the perceived similar pronunciation of both words. A well known example are the Pennsylvania Dutch, who are not of Dutch descent but originally German immigrants, and speak not Dutch but a German dialect and refer to themselves as Pennsilfaanisch Deitsch.
From Old Dutch *thiudisk a southern variant duutsc and a western variant dietsc developed in Middle Dutch. In a broader meaning it could refer to the Germanic as opposed to Romance-language area, while in a narrow sense, duutsc and dietsc referred to the Dutch language. It developed in Modern Dutch in Duyts and Diets respectively and used alongside Nederlands, which referred exclusively to Dutch and would become the modern Dutch autonym, while Duyts (later Duits, though not Diets) would increasingly come to refer to German as opposed to Dutch. During the 16th century, when the process described above had not completed itself, some Dutch linguists and scholars used the term Nederduits/Nederduyts, combining both words, to refer to the Dutch language. A practice which ended in the 19th century but which can still be found in Dutch Reformed Church in South Africa which calls itself the "Nederduitse Gereformeerde Kerk". Nederduits is to be translated as "Dutch" in English, not as "Low German" which would be the literal translation in Modern Dutch.
The Old English variant þeodisc was influenced by the Middle Dutch form of duutsc/dietsc during the Middle Ages and was then mainly used to refer to the inhabitants of the Low Countries, with whom there was extensive trade. Though initially also encompassing a broader definition signifying "speakers of Germanic languages on the continent / Non-Romance speakers" the use of the word Dutch in this sense was rare and would become obsolete following the growing and intensive rivalry between England and the Dutch Republic during the 17th century.