  We report the single-crystal growth and characterization of a new hexagonal
phase, Cu$_{0.82}$Mn$_{1.18}$As, in the Cu-Mn-As system. This compound contains
the same square-pyramidal MnAs$_5$ units as the tetragonal and orthorhombic
polymorphs of CuMnAs. Calorimetry, magnetometry, and neutron diffraction
measurements reveal antiferromagnetic ordering at 270 K. The magnetic structure
consists of a triangular arrangement of spins in the $ab$ plane. Hexagonal
Cu$_{0.82}$Mn$_{1.18}$As shows resistivity that varies only weakly from 5 K to
300 K, and is many times higher than tetragonal CuMnAs, indicative of a
strongly-scattering metal. First-principles calculations confirm the metallic
band structure with a small density of states at the Fermi energy. The
neutron-refined magnetic ground state is close to the
computationally-determined minimum energy configuration. This compound should
serve as a clear control when disentangling the effects of current-driven
N\'{e}el switching of metallic antiferromagnets since it exhibits in-plane
spins but the magnetic ordering does not break degeneracy along the $a$ and $b$
directions, unlike tetragonal CuMnAs.
