摩訶止觀 4.16

4.16

不破者，即是持於十三，無有破損，故名「不破」。若毀犯者，如器破裂也。

The unbroken precept means upholding the thirteen [saṃghāvaśeṣa offenses] without any breakage or damage; hence it is called 'unbroken'. If one violates them, one is like a vessel that has cracked.