He told her all his heart, and said to her, "No razor has ever come on my head; for I have been a Nazirite to God from my mother's womb. If I am shaved, then my strength will go from me, and I will become weak, and be like any other man."
The fact that Samson told Delilah to do something to his hair (Judges 16:13) suggests that he was giving her a clue to his strength. She did not pick this up but kept hounding him for his secret. Finally he gave in (Judges 16:17; cf. Judges 14:17).