  We have measured and analyzed the dielectric constant of the dimer Mott
insulator $\kappa-$(BEDT-TTF)$_2$Cu$_2$(CN)$_3$, which is known as a playground
for a spin-liquid state. Most unexpectedly, this particular organic salt has
nontrivial charge degrees of freedom, being characterized by a
dipole-glass-like dielectric relaxation below around 60 K. This is ascribed to
the charge disproportionation within the dimer due to the intersite Coulomb
repulsion. In this respect, the dipole observed here is purely quantum
mechanical, and thereby exhibits unexpected responses.
