'unloseable' originates from English, specifically formed by the negative prefix 'un-' + the verb 'lose' + the adjectival suffix '-able', where 'un-' meant 'not', 'lose' comes from Old English 'losian' meaning 'to perish / to be lost', and '-able' meant 'capable of'.

'lose' changed from Old English 'losian' (and related Proto-Germanic roots) into Middle English 'losen/losen' and eventually the modern English 'lose'; the adjective was created by adding 'un-' and '-able' to produce 'unloseable' as a nonce or derived formation.

Initially, the root 'lose' meant 'to perish or be lost'; over time, with the addition of 'un-' and '-able', the compound phrase evolved to mean 'not able to be lost' (i.e., incapable or highly unlikely to be lost) in modern usage.