'magisterial' originates from Latin, specifically the word 'magister', where 'magister' meant 'master, teacher', with the adjectival suffix '-ialis' forming 'magisterialis'.

'magister' changed into Medieval Latin 'magisterialis' (or 'magisterialis' attested in late Latin), and this entered Middle English as 'magisterial' via learned/Latin usage.

Initially it meant 'pertaining to a master or magistrate', but over time it evolved into its current senses of 'showing authoritative command' and, by extension, 'imperiously authoritative' in tone.