'un-arranged' is formed in Modern English by the negative prefix 'un-' (Old English 'un-' meaning 'not') attached to the past-participle/adjective form 'arranged' (from the verb 'arrange').

'arrange' comes from Middle French 'arranger' (to set in order), ultimately from Old French 'arangier'/'arrengier', from Vulgar Latin/Frankish influences (a- from Latin ad- 'to/toward' + a Germanic root related to 'rank' or 'row'). The modern compound 'un-arranged' is a straightforward negation formed in later English by adding 'un-' to 'arranged'.

Originally, 'arrange' meant 'to put in a row or order'; 'arranged' retained that meaning as 'put in order', and 'un-arranged' developed as the direct negation meaning 'not put in order' or 'not planned'.