'unreleasable' originates from the prefix 'un-' (Old English) + 'releasable', specifically formed from 'release' + the adjective-forming suffix '-able'. 'un-' meant 'not', 'release' meant 'to set free or let go', and '-able' meant 'capable of'.

'release' changed from Old French words (e.g. 'releser' / 'relaisser') and ultimately from Latin roots (related to 'relaxare' meaning 'to loosen'), entered Middle English (e.g. 'relesen') and became modern English 'release'. The adjective 'releasable' was formed with '-able', and later the negating prefix 'un-' produced 'unreleasable' in modern English.

Initially the root sense was more physical — 'to loosen or set free' — but over time 'release' broadened to include making information or material available; 'unreleasable' now commonly means 'cannot be released' either physically or publicly.