The word 'undiagnosed' is a modern English formation built upon the verb 'diagnose'. 'Diagnose' itself entered English via Latin from the Greek diagignōskein, meaning "to distinguish" or "to discern," composed of dia- (through, thoroughly) and gignōskein (to know). The prefix un-, a common Germanic negation, was attached to the past participle form 'diagnosed' to create an adjective. Thus, the literal construction "not having been thoroughly known (in a medical sense)" evolved into the precise modern meaning of a medical condition that has not yet been identified or classified by a professional.

Structure: un (not) + diagnose (identify a disease) + ed (adjective suffix) - un: A prefix of Germanic origin (Old English) meaning "not," used to form negatives. - diagnose: A verb of Greek origin, from dia- (through, apart) + gignōskein (to know, recognize). In English, it means to identify the nature of an illness. - ed: A suffix of Germanic origin (Old English) used to form adjectives from verbs, indicating a state or condition.