<p>Heavy-fermion materials are mostly rare-earth or actinide intermetallics with very few exceptions in <i>d</i>-electron systems whose underlying physical mechanism remains unclear. Here by studying the quadruple-perovskite CaCu₃Ir₄O₁₂, we propose a symmetry-prohibition mechanism that may enforce heavy-fermion physics in <i>d</i>-electron systems. We show that electron hoppings between nearest- and next-nearest-neighbor Cu-3<i>d</i>_{<i>x</i><i>y</i>} orbitals are strictly prohibited by the crystal symmetry despite their short distances, so that Cu-3<i>d</i>_{<i>x</i><i>y</i>} electrons can only become delocalized through hybridization with the more itinerant Ir-<i>a</i>_1<i>g</i> orbitals, resembling that in typical heavy-fermion materials. Additionally, because <i>d</i> orbitals are much closer to the Fermi level than usual rare-earth intermetallics, the system may be easily tuned to yield rich ground states by chemical substitutions. Our discovery provides a useful way to enforce heavy-fermion physics beyond the Hill rule and may help future material design with highly adaptive properties.</p>