'posteriorly' originates from Latin, specifically the adjective 'posterior', where 'posterior' is the comparative of 'posterus' and meant 'coming after' or 'later'.

'posterior' came into English via Middle English (from Old French/Latin influence) as 'posterior' or forms like 'postereor', and the adverbial form developed into the modern English 'posteriorly'.

Initially it meant 'coming after' (in time or place) and over time it has retained that core sense while being used both spatially ('toward the rear') and temporally ('subsequently').