'just-used' is a compound of the adverb 'just' and the past participle 'used' (from the verb 'use'). 'just' originates from Old French 'juste' and ultimately Latin 'iustus' meaning 'just/right'; 'use' originates from Latin 'uti' via Old French 'user'.

'used' developed as the past participle of Old French 'user' and Middle English 'usen', deriving from Latin 'uti'. 'just' shifted in Middle English from senses related to 'right/fair' (from Latin 'iustus') and also developed the adverbial sense 'a short time ago', producing the modern combination 'just used'.

Initially 'just' meant 'right, fair' (from Latin 'iustus') but later gained the adverbial sense 'a short time ago'; combined with 'used', the phrase came to mean 'used a short time ago'.