'apportion' originates from Middle French, specifically the word 'apportioner', where the prefix 'a-' (from Latin 'ad-') meant 'to/toward' and the root 'portio' (from Latin) meant 'a share or part'.

'apportion' changed from Middle French 'apportioner' and Old French forms and eventually became the modern English word 'apportion' via Middle English borrowings.

Initially, it meant 'to assign a portion or share', and over time it has retained this core meaning while extending to contexts like assigning responsibility or costs.