  We report the detection of a massive neutral gas outflow in the z=2.09
gravitationally lensed Dusty Star Forming Galaxy HATLASJ085358.9+015537
(G09v1.40), seen in absorption with the OH+(1_1-1_0) transition using spatially
resolved Atacama Large Millimeter/submillimeter Array (ALMA) observations. The
OH+ line, tracing diffuse atomic gas, is observed simultaneously with the
CO(9-8) emission line and underlying dust continuum, providing kinematic and
morphological information of the warm dense gas and dust in the host galaxy,
respectively. Source plane reconstruction reveals a compact nuclear dust region
from which the conical OH+ outflow emerges, away from the more extended and
offset CO(9-8) and stellar distributions. The total atomic gas mass of the
observed outflow is 6.7x10^9 M_sun>25 as massive as the molecular gas component
within the galaxy. We derive outflow properties over a range of possible
conical outflow inclinations (0.38 deg-64 deg), finding a neutral gas mass
outflow rate between 83-25400 M_sun/yr, exceeding the star formation rate (788
M_sun/yr) if the inclination is >3.6 deg (mass-loading factor = 0.3-4.7).
Kinetic energy and momentum fluxes are within 4.4-290x10^9 L_sun and
0.1-3.7x10^37 dyne, respectively (energy-loading factor = 0.013-16). If the
inclination is small (<12 deg), star formation may be responsible for driving
the neutral outflow via energy- or momentum-driven scenarios, depending on its
orientation. If the inclination is >12 deg, unusually high coupling
efficiencies to the ISM, or an additional energy source (e.g. an active
galactic nucleus), is needed.
