I obviously know the history behind 'Weimar' (It being the place where the assembly met and the constitution was drafted) but when did the term became common in usage? I heard that Hitler popularized it late in its short existence?
For me it is easy to imagine that a national assembly which had to relocate because of popular uprisings would be ridiculed for that. Based on that assumption it isn't far to guess that the term always had a negative connotation.
But today it is used interchangeably without major differences on all ends of the political spectrum. Maybe someone can enlighten me?
The name "Weimar Republic" was not really used much during the existence of the Republic. The Republic used "Deutsches Reich" as the official name, and "Republik" in various compounds, but Weimar was an informal title at best. Some Anglophone publications used Weimar Republic yet this was typically a minor naming convention. The actual town of Weimar had a rather strained relationship with the Republic. It was the site of the Republic's constitutional convention, chosen mostly because it was away from the chaos of Berlin, and the was the center for Gropius's Bauhaus movement. Yet the largely conservative Weimar population had little love for the Republic and many locals resented the Bauhaus for bringing in foreigners and Jews into Goethe's birthplace. Hitler and the NSDAP though were the ones who popularized the name Weimar Republic, and he and other NSDAP propagandists used it as a slur. The name stuck after 1933 and filtered into opponents of the regime as well as outside of Germany. This google Ngram for English shows the post-1930 pickup, as does this Ngram for "Weimarer Republik." As both Ngrams indicate, the Weimar Republic clearly became the "Weimar Republic" after 1945.
The fact that historians and other writers use Weimar Republic when the actual government in question did not is not terribly unusual. There are other examples of Anglophone names that are anachronistic or inaccurate. The Polish-Lithuanian Commonwealth is one, as is the Byzantine Empire. One notable example that sometimes strikes pedants is the use of Tsar after Peter I. Peter I accepted the title All-Russian Imperator in 1721 and it became the first title used by subsequent Romanovs. This was part of Peter I's efforts at building up his prestige and self-styling as a modern European monarch who had no peers. The tsar title implied he was still Muscovite, but imperator is a more ambitious. Subsequent Romanovs would use the imperial title as their main title, and tsar became relegated to various monarchical appendages that signified areas over which the tsar had suzerainty, such as the Tsar of Kazan, Tsar of Siberia, etc. The title Tsar also entered into the vernacular as a shorthand for the Russian ruler in the non-Russian world, but subsequent emperors after Peter I sometimes preferred Tsar over Emperor, such as the archconservative Alexander III.