'parenthetically' originates from English, specifically the adjective 'parenthetical', where 'parenthetical' ultimately derives from Greek 'parenthesis' via Latin, with the Greek elements 'para-' meaning 'beside' and 'tithenai' meaning 'to place'.

'parenthetical' changed from Greek 'parenthesis' to Latin 'parenthesis', entered Middle English as 'parenthesis', developed into the adjective 'parenthetical' in Modern English, and the adverb 'parenthetically' was formed from that adjective by adding the suffix '-ly'.

Initially it referred to 'a placing beside' or an insertion (a literal parenthesis), but over time it evolved into its current meanings of 'in an inserted or incidental manner' and 'as an incidental remark'.