'aru' originates from Proto-Japonic (or Old Japanese roots), specifically the root '*ar(ə)' meaning 'to be/exist'.

'aru' appears in Old Japanese as 'aru' (or underlying root 'ar-') and has continued into Modern Japanese as 'aru' with largely the same form.

Initially, it meant 'to be' or 'to exist', and over time it has retained that core meaning while extending to indicate possession and functioning as a prenominal determiner ('a certain').