in secret; secretly; confidentially
Used to indicate that something is done or kept hidden from others. Often appears in formal or written contexts.
その計画は秘中に進められた。
The plan was carried out in secret.
彼は秘中に情報を集めていた。
He was gathering information secretly.
Compound of 秘 (secret) and 中 (inside, among), literally 'within secrecy'. The exact historical derivation is uncertain, but the meaning is transparent from the kanji.