This proposal aims to experimentally characterise the anisotropic lattice dynamics of the most stable allotrope of phosphorus, i.e. the monoclinic violet phosphorus, through a combination of x-ray diffuse scattering and inelastic x-ray scattering. This layered crystal has very recently emerged thanks to its favourable semiconducting properties and improved stability with respect to black phosphorus. From the phonon dispersions at terahertz frequencies, we plan to derive the macroscopic elastic properties of violet phosphorus and its elastic moduli. The elastic stiffness coefficients obtained here provide a fundamental benchmark for models of mechanical and thermodynamical properties of this yet unexplored layered allotrope of phosphorus and its related two-dimensional crystals, such as violet phosphorene, which is a promising quantum materials disclosing a huge potential for nanophotonics, optoelectronics.