adverb of time, Old English þanne, þænne, þonne, from Proto-Germanic *thana- (source also of Old Frisian thenne, Old Saxon thanna, Dutch dan, Old High German danne, German dann), from PIE demonstrative pronoun root *to- (see the).
As a conjunction, "in that case, therefore," in Old English. As an adjective, "being at that time," from 1580s. As a noun from early 14c. For further sense development, see than. Similar evolutions in other Germanic languages; Dutch uses dan in both senses, but German has dann (adv.) "then," denn (conj.) "than." Now and then "at various times" is attested from 1550s; earlier then and then (c. 1200).