something concealed; something not disclosed
Refers to a matter that is kept hidden or not revealed. Often used in formal or written contexts.
It seems he has something he is not disclosing.
不告のままでは信頼を失う。
If you keep things concealed, you will lose trust.
Compound of 不 (fu, 'not') and 告 (koku, 'to inform'). The exact historical derivation is uncertain, but the meaning is transparent from the kanji.