Early 16th century; earliest use found in Gavin Douglas (c1476–1522), poet and bishop of Dunkeld. From un- + returnable, in sense 1 originally after classical Latin irremeābilisirremeable.
Definition of unreturnable in US English:
unreturnable
adjective(ˌ)ʌnrɪˈtəːnəbl
1Allowing no return.
2That cannot be returned.
Origin
Early 16th century; earliest use found in Gavin Douglas (c1476–1522), poet and bishop of Dunkeld. From un- + returnable, in sense 1 originally after classical Latin irremeābilis irremeable.