Binary non-collinear incommensurate SDW compound FeAs may represent a unique itinerant antiferromagnet, not only different from its pnictide superconductor cousins, but also distinct from other canonical itinerant antiferromagnets like MnSi and Cr. Therefore, it would be crucial to determine magnetic interactions by inelastic neutron scattering in order to pin down the microscopic origin of this peculiar magnetic ground state. Unfortunately, this kind of studies has not been carried out up to now. Here we propose to carry out the first inelastic neutron scattering experiment on a high-quality FeAs single crystal (~8 g) at Merlin. We believe that the planned experiment will not only help to gain microscopic understanding on this fascinating itinerant magnet, but may also provide new insights on the nature of magnetism of the iron based superconductors.