An Etymological Dictionary of the German Language/Annotated/dann
dann, adverb, from the equivalent Middle High German and Old High German danne, ‘then, at that time, in such a case, thereupon’; properly identical with denn; in Middle High German and Old High German danne is used indifferently for denn and dann. Anglo-Saxon þonne, þœnne, English then. The Old Teutonic adverb is based on the pronominal stem þa- (compare der); yet the mode of its formation is nor quite clear. Compare da, der, and the following word.