'unappealable' is formed by prefixing 'un-' to 'appealable' (itself 'appeal' + suffix '-able'), where 'un-' means 'not' and '-able' means 'capable of or suitable for'.

'appeal' comes via Anglo-French 'apeler'/'appel' and Old French from Latin 'appellare' meaning 'to address or call upon'; over time 'appeal' acquired a legal sense of applying to a higher court, and from that 'appealable' (able to be appealed) was formed; 'unappealable' is the modern negative formation.

Initially 'appellare' meant 'to address or call upon'; over time the English 'appeal' developed a legal sense of seeking review by a higher court, and 'unappealable' now means 'not able to be appealed' in that legal sense.