To create self-controlled radiation photonics systems, it is necessary to have complete information about the nonlinear properties of the materials used. In this Letter, the vibrational mechanism of the giant low-inertia cubic nonlinearity of the refractive index of water in the terahertz (THz) frequency range is experimentally proven. Its dominance, which manifests itself when the temperature of the liquid changes, is demonstrated. The measured nonlinear refractive index in the THz frequency range for a water jet at temperatures from 14°C to 21°C demonstrates a correlation with the theoretical approach, varies in the range 4-10 × 10-10 cm2/W, and is characterized by an inertial time constant of less than 1 ps.