'nonpublicly' originates from Modern English, specifically formed from the negative prefix 'non-' (from Latin 'non', meaning 'not'), the adjective 'public' (from Latin 'publicus'), and the adverbial suffix '-ly' (from Old English '-lic', later '-ly').

'public' came into English via Latin 'publicus' through Old French 'publique' and Middle English; the compound 'non-public' (negative + adjective) was later given the adverbial ending '-ly' to form 'non-publicly', with the closed form 'nonpublicly' appearing in Modern English.

Initially the parts meant 'not' + 'of the people/public'; over time the combined form came to mean 'not in public; privately' and is used to indicate actions or information that are not made public.