The word 'unfamiliar' is a straightforward English formation built upon the well-established adjective 'familiar'. 'Familiar' itself traces back to the Latin familiaris, which described something pertaining to the familia, the household or family unit, thus carrying connotations of intimacy and close knowledge. The Germanic prefix 'un-', one of the most common negating elements in English, is attached to directly invert the meaning. Therefore, the literal construction "not of the household" logically evolved into the modern abstract sense of "not known, recognized, or within one's experience."

Structure: un (not) + familiar (well-known) - un: A prefix of Germanic origin (Old English, Old High German, Old Norse) meaning "not," used to form negatives of adjectives and participles. - familiar: An adjective from Latin familiaris, meaning "of a household, domestic, intimate," derived from familia ("household, family").