'rhetorically' originates from Greek, specifically the word 'rhētorikos', where 'rhētor' meant 'public speaker' and '-ikos' meant 'pertaining to'; it entered English via Latin 'rhetoricus' and Middle French/Medieval Latin forms.

'rhetorically' changed from Greek 'rhētorikos' to Latin 'rhetoricus', passed through Old/Middle French and Late Latin forms (e.g. 'retoricus'/'rhetoricus') and eventually became the Modern English 'rhetorical' and its adverbial form 'rhetorically'.

Initially it referred specifically to matters of public speaking or oratory; over time it broadened to mean 'pertaining to rhetorical style' and figuratively 'said for effect, persuasive, or insincere style'.