This is the first time in the narrative that Samson shows an awareness that his strength was contingent on his keeping his Nazirite vow. It is thus not certain how Samson knew that shaving his head would result in a loss of strength. Any proposal would in effect be an argument from silence. But it is important to recall from Numbers 6:1–27 that when a Nazirite whose vow was temporary and voluntary (in contrast to Samson’s permanent and involuntary vow) would complete the period of his vow, he would undergo a ceremony to terminate his status as a Nazirite. This ceremony involved the shaving of his consecrated head at the entrance of the tent of meeting, whereupon the shorn locks, holy as they were, were not discarded but burned (Numbers 6:18–20). Perhaps Samson would have understood from this that the termination of a Naziriteship through the shaving of hair meant the termination of one’s consecration to God, such that God’s special gifts would also depart from him, including his unique strength, and make him weak like all men. Again, we cannot be certain that this is informing Samson. In the end we have to work with what the author tells us.
17 And he told her all his heart, and said to her, “A razor has never come upon my head, for I have been a Nazirite to God from my mother’s womb. If my head is shaved, then my strength will leave me, and I shall become weak and be like any other man.”