'unimpressive' originates from the prefix 'un-' meaning 'not' and the word 'impressive' from Latin 'impressivus', where 'im-' meant 'into' and 'press' meant 'to press'.

'impressivus' transformed into the French word 'impressif', and eventually became the modern English word 'impressive'.

Initially, it meant 'capable of being impressed', but over time it evolved into its current meaning of 'not making a strong impression'.