'unillustrated' originates from English, formed by the prefix 'un-' + the past participle 'illustrated', where 'un-' meant 'not' and 'illustrated' comes from the verb 'illustrate'.

'illustrate' comes from Latin 'illustrare' (to make light, brighten) via Old French and Middle English; 'illustrated' is the past participle form and 'un-' was prefixed in Modern English to create 'unillustrated'.

Originally 'illustrate' (Latin root) carried the sense 'to illuminate or make clear'; over time in English it also took on the sense 'to provide with pictures', and 'unillustrated' has come to mean 'not provided with pictures'.