na-adjective, noun
dissent; disagreement; refusal
Formal noun/adjectival noun expressing strong objection, disapproval, or refusal. Often used in official or serious contexts.
彼の提案に不承知の意を示した。
I expressed my dissent to his proposal.
不承知ならば、はっきりそう言ってください。
If you disagree, please say so clearly.
拒否 is a direct refusal or rejection, often used in legal or formal contexts. 不承知 implies a stance of not consenting, which may be less absolute.
Compound of 不 (negative prefix) and 承知 (consent, knowledge). The exact historical derivation is uncertain, but the word has been used in formal Japanese to express non-consent.