'seditious' originates from Latin, specifically the word 'seditiosus' (from 'seditio'), where 'seditio' meant 'a going aside, dissension, insurrection'.

'seditious' entered English via Old French/Anglo-French (e.g. Old French 'seditieux') and Middle English forms, eventually becoming the modern English adjective 'seditious' from which 'seditiously' is formed by adding the adverbial suffix '-ly'.

Initially it referred to 'dissension' or 'a tendency to cause dissent/insurrection'; over time it retained that sense and came to specifically mean 'tending to incite rebellion or unlawful resistance' in modern usage.