  We present Atacama Large Millimeter/submillimeter Array observations of a
massive (M_stars~10^11 M_Sun) compact (r_e,UV~100 pc) merger remnant at z=0.66
that is driving a 1000 km/s outflow of cool gas, with no observational trace of
an active galactic nucleus (AGN). We resolve molecular gas on scales of
approximately 1-2 kpc, and our main finding is the discovery of a wing of
blueshifted CO(2-1) emission out to -1000 km/s relative to the stars. We argue
that this is the molecular component of a multiphase outflow, expelled from the
central starburst within the past 5 Myr through stellar feedback, although we
cannot rule out previous AGN activity as a launching mechanism. If the latter
is true, then this is an example of a relic multiphase AGN outflow. We estimate
a molecular mass outflow rate of approximately 300 M_Sun/yr, or about one third
of the 10 Myr-averaged star formation rate. This system epitomizes the
multiphase 'blowout' episode following a dissipational major merger - a process
that has violently quenched central star formation and supermassive black hole
growth.
