  Mott insulators are commonly pictured with electrons localized on lattice
sites. Their low-energy degrees of freedom involve spins only. Here we observe
emerging charge degrees of freedom in a molecule-based Mott insulator
$\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Br, resulting in a quantum dipole liquid
state. Electrons localized on molecular dimer lattice sites form electric
dipoles that do not order at low temperatures and fluctuate with frequency
detected experimentally in our Raman spectroscopy experiments. The heat
capacity and Raman scattering response are consistent with a scenario where the
composite spin and electric dipole degrees of freedom remain fluctuating down
to the lowest measured temperatures.
