What did Samson finally tell Delilah was the secret of his strength?

Answer: His hair had never been cut

Samson said he had been a Nazirite from his mother's womb: "If I am shaved, my strength will leave me."

“Samson told her all that was in his heart: “My hair has never been cut, because I have been a Nazirite to God from my mother’s womb. If I am shaved, my strength will leave me, and I will become as weak as any other man.””— Judges 16:17 (BSB)