Secret
Confidential
noun, の adjective
Keep information inside a cord that cordons off just you and a few others (or maybe no one else). Only the people you let inside the cord can know, because it's a secret and confidential.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
私はICレコーダーで会話を内緒で録音します。
I secretly record conversations with my IC recorder.