'preassign' originates from Latin elements: the prefix 'pre-' (from Latin 'prae') meaning 'before' and the verb 'assign' from Latin 'assignare'.

'assign' came into English via Old French (e.g. 'asignier') from Latin 'assignare'; the English formation 'pre-' + 'assign' produced 'preassign' to mean 'assign beforehand', and the derived past/p.p. 'preassigned' follows usual English verb inflection.

Initially it meant 'to assign beforehand' and this core meaning has remained essentially the same in modern usage as 'assign in advance' or 'allocated ahead of time'.