'posthumously' originates from Late Latin, specifically from 'posthumus' (itself from Latin 'postumus'), where the prefix 'post-' meant 'after' and a later folk association linked it with Latin 'humus' meaning 'ground'.

'posthumously' changed via Old French 'posthume' and Middle English 'posthumous' and eventually became the modern English adverb 'posthumously' (adjective + suffix '-ly').

Initially related to 'postumus' meaning 'last' (in sense of 'born after the father's death' or 'last-born'), but over time it evolved into the sense 'after death' and now commonly means 'after the death of the person mentioned'.