'unallocatability' is formed in English by adding the prefix 'un-' (meaning 'not') to 'allocatability', which itself comes from 'allocate' + '-ability'. 'Allocate' comes from Latin 'allocare', where 'ad-' meant 'to' and 'locare' meant 'to place'.

'Allocatability' was formed from 'allocate' + '-ability' in modern English, and 'unallocatability' was later formed by adding 'un-'. 'Allocate' came from Latin 'allocare', which passed through Middle French and then into English.

Initially, the root 'allocate' meant 'to assign or distribute', and 'allocatability' meant 'the ability to be allocated'. Adding 'un-' changed the meaning to 'the inability to be allocated'.