confidentially; privately; off the record
Used when information is shared in confidence, not for public knowledge. Often used in business or personal contexts to indicate secrecy or discretion.
Please keep this matter confidential.
Please handle this discreetly.
内密に means 'secretly/confidentially', the direct opposite of 表立って, which describes doing something openly.
Compound of 内 (inside, private) and 密 (secret, dense), with the adverbial particle に. The exact historical derivation is uncertain, but the combination naturally conveys 'in a private and confidential manner'.