kosotto
Moving or acting quietly and stealthily so as not to be noticed. A single, discrete sneaky action.
💡Sneaking a cookie — 'She kosotto grabbed a cookie when no one was looking.'
こそっと部屋を抜け出した。
I kosotto slipped out of the room.
He ___ slipped a note into her bag without anyone noticing.
彼は誰にも気づかれずに○○と彼女のカバンにメモを入れた。