Secret, Confidential
noun, の adjective
We're inside together on this information. Just you and me, inside our knowledge. It is secret and confidential.
ないしょ
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.