The Kitaev model, a honeycomb network of spins with bond-dependent anisotropic interactions, is a rare example of a system with a quantum spin liquid ground state. Although most Kitaev model candidate materials eventually order magnetically due to additional non-Kitaev interactions, their bond-dependent anisotropy manifests in unusual spin dynamics. Recent research suggests that bond-dependent anisotropy can stabilize exotic magnetic phases on the geometrically frustrated triangular lattice. Unfortunately, few materials have been identified with simultaneous geometric frustration and bond-dependent anisotropy. Here, we report a frustrated triangular lattice with bond-dependent anisotropy in the cobalt-based van der Waals antiferromagnet CoI2. Momentum and energy-resolved inelastic neutron scattering measurements show substantial magnon decay and level repulsion. A thorough examination of excitations in both the paramagnetic and magnetically ordered states demonstrates that the bond-dependent anisotropy is the origin of the spiral order and the magnon decay found in CoI2. Our results provide the basis for future studies of the interplay between Kitaev magnetism and geometric frustration.
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