Quietly or secretly; done stealthily so as not to be noticed by others.
彼女はパーティーにこっそり来て、端の席に座った。
She slipped into the party quietly and sat at a corner seat.
試験中にこっそりノートを見てはいけません。
You must not secretly look at your notes during the exam.
新製品の情報は社内でこっそり共有されていた。
Information about the new product was being shared discreetly within the company.