The magnetic subsystem of nabokoite, KCu(TeO)(SO)Cl, is constituted by buckled square kagome lattice of copper decorated by quasi-isolated Cu ions. This combination determines peculiar physical properties of this compound evidenced in electron spin resonance (ESR) spectroscopy, dielectric permittivity , magnetization and specific heat measurements. At lowering temperature, the magnetic susceptibility passes through broad hump at about 150 K inherent for low-dimensional magnetic systems and evidences sharp peak at antiferromagnetic phase transition at K. The curve also exhibits sharp peak at readily suppressed by magnetic field and additional peak-like anomaly at K robust to magnetic field. The latter can be ascribed to low-lying singlet excitations filling the singlet-triplet gap in magnetic excitation spectrum of the square kagome lattice [J.Richter, O.Derzhko and J.Schnack, Phys. Rev. B 105, 144427 (2022)]. According to position of , the leading exchange interaction parameter in nabokoite is estimated to be about 60K. ESR spectroscopy provides indications that antiferromagnetic structure below is non-collinear. These complex thermodynamic and resonant properties signal the presence of two weakly coupled magnetic subsystems in nabokoite, namely spin-liquid with large singlet-triplet gap and antiferromagnet represented by decorating ions. Separate issue is the observation of antiferroelectric-type behavior in at low temperatures, which tentatively reduces the symmetry and partially lifts frustration of magnetic interactions of decorating copper ions with buckled square kagome lattice.