'unjustly-treated' originates from English, specifically combining the prefix 'un-' meaning 'not' and the adjective 'just' (from Latin 'iustus' via French) with the adverbial suffix '-ly' to form 'unjustly', and the past participle 'treated' from Old French 'traiter' (from Latin 'tractare') meaning 'to handle'.

'tractare' transformed into Old French 'traiter', which became Middle English 'treaten'/'tret' and then modern English 'treat'; 'just' came via French from Latin 'iustus'; the prefix 'un-' came from Old English 'un-'. These elements combined in English to form the attributive compound adjective 'unjustly-treated'.

Initially, it meant 'handled or dealt with in a way that is not fair or just,' and this meaning has remained stable in modern usage.