  We investigate low-temperature spin correlations in the metallic frustrated
magnet \beta-MnCo. Single-crystal polarised-neutron scattering experiments
reveal the persistence of highly-structured magnetic diffuse scattering and the
absence of periodic magnetic order to T=0.05 K. We employ reverse Monte Carlo
refinements and mean-field theory simulations to construct a simple effective
Hamiltonian which accounts for the magnetic scattering. The interactions we
identify describe an emergent spin structure which mimics the triangular
lattice antiferromagnet. The observation of a simple collective magnetic state
in a complicated crystal structure is surprising because it reverses the
established paradigm of elaborate emergent states arising from many-body
interactions on simple lattices. We suggest that structural complexity may
provide a route to realising new states of correlated quantum matter.
