'magnetically' originates from Modern English, specifically the adjective 'magnetic' plus the adverbial suffix '-ally'. 'Magnetic' ultimately comes from New Latin 'magneticus'.

'magnetically' changed from Middle English 'magnetic' (from Old French 'magnétique' and Medieval/New Latin 'magneticus'), which in turn derives from Greek 'magnētēs' (μαγνήτης) meaning 'lodestone' or 'magnet'. The adverbial form was formed by adding '-ally' to 'magnetic'.

Initially it meant 'of or pertaining to a lodestone or magnet' and retained that literal sense; later the adverb also acquired a figurative sense of 'irresistibly attractive'.