We report to be an improper multiferroic with the simultaneous development of electric polarization and magnetization below . The observed spontaneous polarization of magnitude is highest among copper-based improper multiferroic materials. Our study demonstrates a sizable amount of magnetoelectric coupling below , even with a low magnetic field. The theoretical calculations based on density functional theory indicate magnetism in is a consequence of ferro-orbital ordering driven by a polar lattice distortion due to the unique pyramidal () environment of Cu. Spin-orbit coupling further stabilizes orbital ordering and is crucial for magnetism. The calculations indicate that the origin of the giant ferroelectric polarization is primarily due to the symmetric exchange-striction mechanism and is corroborated by temperature-dependent x-ray studies.