The frequency-dependent self-susceptibility of a selected particle in the Drude-Lorentz model of a nonpolar fluid is shown to have a universal absorption spectrum in the limit of low density. We derive an expression for the universal lineshape and calculate the limit function by a scaling procedure, in a self-consistent ring approximation. The line shape is found to be an asymmetric function of frequency, as suggested by computer simulation data for systems of hard spheres and of atoms with Lennard-Jones interactions. A calculation of correction terms accounting for the effect of the microstructure at moderate density leads to fairly good agreement with existing computer simulation data.