secret reason
noun phraseUK /ˈsiː.krət ˈriː.zən/
US /ˈsiː.krət ˈriː.zən/
A reason you keep hidden and do not tell anyone.
She smiled for a secret reason that nobody else knew.
OTHER MEANINGS
2. A cause that is not openly stated but explains someone's actions.
The detective guessed there was a secret reason behind his sudden move.
3. An explanation kept private to protect someone's feelings.
He gave a secret reason for skipping the party so no one would feel bad.
4. A hidden purpose behind a plan or decision.
The surprise gift had a secret reason: to say thank you.