'unlosable' originates from English, specifically formed by the prefix 'un-' (from Old English 'un-' meaning 'not') plus 'losable', which comes from the verb 'lose' (Old English 'lēosan'/'losian') with the adjectival suffix '-able'.

'lose' comes from Old English 'lēosan' (or 'losian'), from Proto-Germanic roots meaning 'to lose'; the suffix '-able' derives from Old French '-able' and ultimately Latin '-abilis'; the modern English formation 'un-' + 'losable' produced 'unlosable'.

Initially the root meant 'to lose' in Old English; the modern compound 'unlosable' has the straightforward current meaning 'not able to be lost' (and by extension, sometimes 'certain to win').