Origins
The word 'German' is a case study in how a people can be named by outsiders, with the name persisting for millennia even though those people have never used it for themselves.
The English word 'German' comes from Latin 'Germānus,' which the Romans used to designate the peoples living east of the Rhine and north of the upper Danube. The earliest surviving use of the term is in Julius Caesar's 'De Bello Gallico' (Commentaries on the Gallic War), written in the 50s BCE, where he distinguishes the 'Germani' from the 'Galli' (Gauls) as a separate group of peoples. Caesar appears to have adopted the term from the Gauls themselves, who already used some form of it to refer to their eastern neighbors.
The etymology of 'Germānus' is one of the most debated questions in European historical linguistics, with no scholarly consensus even after centuries of inquiry. The main competing theories are:
Early History
First, a Celtic origin meaning 'neighbor.' This connects the word to Old Irish 'gair' ('near') and proposes that the Gauls simply called the peoples across the Rhine 'the neighbors.' This is currently the most widely favored theory among specialists, partly because it explains why the term originated among Celtic speakers rather than the Germanic peoples themselves.
Second, a Celtic origin meaning 'noisy ones' or 'shouters,' from a root related to Old Irish 'garim' ('I shout, I call'). This would have been a characterization of the Germanic peoples by their Celtic neighbors, perhaps referring to their war cries or their unintelligible speech.
Third, a Germanic self-designation meaning 'spear-men,' from Proto-Germanic *gaizaz ('spear,' cf. Old High German 'gēr,' Old English 'gār,' which survives in names like 'Roger' and 'Edgar'). Under this theory, 'Germani' would originally have been the name of a specific tribe that was then extended by outsiders to the larger group.
Latin Roots
Fourth, a connection to Latin 'germānus' meaning 'genuine, authentic' (as in 'germane'), suggesting the Romans saw these peoples as the 'true' or 'original' barbarians. This folk etymology is generally rejected by modern scholars as a coincidental Latin homophone.
What makes the naming of Germany especially remarkable is the sheer number of different names that European languages use for the same country and people. The Germans call themselves 'Deutsch' and their country 'Deutschland,' from Proto-Germanic *þeudiskaz, meaning 'of the people' (from *þeudō, 'people,' cognate with the Celtic tribe name 'Teutons'). The French call the country 'Allemagne' and its people 'Allemands,' from the Alemanni, a confederation of Germanic tribes on the upper Rhine. The Finns and Estonians say 'Saksa,' derived from the Saxons. The Slavic languages use variants of a word meaning 'mute' — Polish 'Niemcy,' Czech 'Německo,' Russian 'Немцы' (Nemtsy) — from the Slavic root *němьcь, meaning 'one who is mute,' i.e., someone who doesn't speak an intelligible language. The Italians use 'Germania' (from Latin), while Latvians say 'Vācija' (possibly from a different tribal name). No other major country has quite so many unrelated exonyms.
In English, 'German' as an adjective and noun dates to the 14th century in the form 'Germain,' though 'Almain' (from the Alemanni, via French) was also common in Middle English and survived in the compound 'Almain rivet' (a type of German-made armor) into the 16th century. The modern form 'German' with its current spelling stabilized in the 1520s.
Later History
The adjective 'Germanic,' used in a broader linguistic and ethnographic sense to encompass all the related peoples and languages (English, Dutch, Scandinavian, etc.), was coined in the 18th century by scholars who needed a term wider than 'German.' The distinction between 'German' (specific to Germany) and 'Germanic' (the whole language family) is a purely English convention that does not exist in German itself, where 'germanisch' and 'deutsch' serve these roles.
Tacitus's 'Germania' (98 CE), one of the most influential ethnographic works ever written, fixed the Roman understanding of the Germanic peoples for centuries. His account — describing them as fierce, freedom-loving, and uncorrupted by civilization — became enormously influential during the Renaissance and was later misused to construct nationalist and racialist ideologies. The word 'German' thus carries not only linguistic history but a heavy freight of political and ideological associations that have accumulated over two millennia.