'unmissable' is formed in modern English from the negative prefix 'un-' + the verb 'miss' + the adjective-forming suffix '-able'.

'miss' comes from Old English 'missan' meaning 'to miss, fail to hit or find'; the suffix '-able' entered English via Old French and Latin (Latin '-abilis'), and the prefix 'un-' is from Old English 'un-' meaning 'not'. These elements combined in Modern English to form 'unmissable'.

The compound originally meant literally 'not able to be missed' and has retained that basic sense; it has also developed a broader idiomatic use meaning 'extremely worthwhile or essential to see/attend'.