The compound adjective "then-unknown" is a modern English formation built from native Germanic elements. The core concept originates from the Old English verb cnāwan, meaning "to know," which is traceable to the Proto-Indo-European root ǵneh₃-, signifying "to know." This root is prefixed with the Old English negative particle un-, a common Germanic negation from PIE n̥-, to create "unknown," meaning "not known." To specify the temporal aspect, the native English adverb "then," derived from Old English demonstrative and temporal forms, is hyphenated to the front. The word's logic is transparently literal, directly combining "at that past time" + "not" + "known" to describe something that was not recognized during the period being discussed.

Structure: then (time) + un (not) + known (recognized) - then: From Old English þanne, þænne, þonne. An adverb used here in a compound to specify a temporal context. - un: From Old English un-, a prefix expressing negation or absence. - known: Past participle of the verb know, from Old English cnāwan (to know, recognize). Functions as the core adjective meaning "recognized or understood."