The word 'undefinable' is a modern English formation built upon the core verb 'define,' which traces back to the Latin 'definire,' meaning to set bounds or limits. The negative prefix 'un-' is added to reverse the meaning, creating the sense of "not capable of being bounded or limited." Finally, the suffix '-able' transforms this concept into an adjective describing a quality. Thus, the literal morpheme combination "not + to limit + capable of" elegantly conveys the abstract idea of something that eludes precise description or categorization.

Structure: un (not) + defin(e) (to state the meaning of) + able (capable of) - un: A prefix of Germanic origin (Old English) meaning "not," used to form negatives. - define: A verb of Latin origin, from 'de-' (completely) + 'finire' (to limit, end). The core sense is to state the precise limits or meaning of something. - able: A suffix of Latin origin (via Old French), from '-abilis,' meaning "capable of, fit for, tending to."