  We report experimental and theoretical evidence that
Rb$_2$Cu$_2$Mo$_3$O$_{12}$ has a nonmagnetic tetramer ground state of a two-leg
ladder comprising antiferromagnetically coupled frustrated spin-$1/2$ chains
and exhibits a Haldane spin gap of emergent spin-1 pairs. Three spin
excitations split from the spin-1 triplet by a Dzyaloshinskii-Moriya
interaction are identified in inelastic neutron-scattering and electron spin
resonance spectra. A tiny magnetic field generates ferroelectricity without
closing the spin gap, indicating a novel class of ferroelectricity induced by a
vector spin chirality order.
