'unostentatious' is formed from the negative prefix 'un-' and the adjective 'ostentatious'. 'Ostentatious' originates from Latin, specifically the word 'ostentātiō' / 'ostentāre', where the root related to 'ostendere' meant 'to show' or 'a showing'.

'ostentatious' changed from Latin 'ostentātiō'/'ostentāre' into Medieval Latin and Old French forms and entered Middle English as 'ostentatious'; the modern English adjective 'unostentatious' was created by adding the prefix 'un-' (negation) to that adjective.

Initially the Latin root conveyed the action 'to show' or 'a showing'; over time 'ostentatious' came to mean 'showy, intended to attract notice', and 'unostentatious' now means 'not showy; modest or restrained in display'.