  We present inelastic neutron scattering (INS) measurements of magnetic
excitations in YbMnBi$_2$, which reveal features consistent with a direct
coupling of magnetic excitations to Dirac fermions. In contrast with the large
broadening of magnetic spectra observed in antiferromagnetic metals such as the
iron pnictides, here the spin waves exhibit a small but resolvable intrinsic
width, consistent with our theoretical analysis. The subtle manifestation of
spin-fermion coupling is a consequence of the Dirac nature of the conduction
electrons, including the vanishing density of states near the Dirac points.
Accounting for the Dirac fermion dispersion specific to \ymb\ leads to
particular signatures, such as the nearly wave-vector independent damping
observed in the experiment.
