Energy-efficient spin-based technologies require new prospects for the realization of nanoscale logic devices that are able to operate at terahertz (THz) frequencies. For instance, in the field of magnonics, spin waves are proposed for data transport and processing. In this regard, magnonic concepts rely on generation of nanometer-wavelength spin waves, confined in a nanostructure. Here, we propose a layered metallic system, based on a ferromagnetic film sandwiched by heavy metals that allows a highly efficient coupling of millimeter wavelength THz light with nanometer-wavelength magnon modes. Using single-cycle broadband THz radiation, we are able to excite spin-wave modes with a frequency of up to 0.6 THz and a wavelength as short as 6 nm. Numerical simulations demonstrate that the coupling originates solely from interfacial spin-orbit torques. A possible origin of the enhanced damping of THz spin waves will be discussed.