'unimpressive' originates from the prefix 'un-' meaning 'not' and the word 'impressive', which comes from Latin 'impressivus', meaning 'capable of being impressed'.

'impressivus' transformed into the French word 'impressif', and eventually became the modern English word 'impressive', with 'un-' added to form 'unimpressive'.

Initially, 'impressive' meant 'capable of being impressed', but over time it evolved into its current meaning of 'evoking admiration'. 'Unimpressive' negates this meaning.