  We present Adaptive Optics assisted near-IR integral field spectroscopic
observations of a luminous quasar at $z = 2.4$, previously observed as the
first known example at high redshift of large scale quasar-driven outflow
quenching star formation in its host galaxy. The nuclear spectrum shows broad
and blueshifted H$\beta$ in absorption, which is tracing outflowing gas with
high densities ($>10^8$ - $10^9$ cm$^{-3}$) and velocities in excess of 10,000
km s$^{-1}$. The properties of the outflowing clouds (covering factor, density,
column density and inferred location) indicate that they likely originate from
the Broad Line Region. The energetics of such nuclear regions are consistent
with that observed in the large scale outflow, supporting models in which
quasar driven outflows originate from the nuclear region and are energy
conserving. We note that the asymmetric profile of both the H$\beta$ and
H$\alpha$ emission lines is likely due to absorption by the dense outflowing
gas along the line of sight. This outflow-induced asymmetry has implications on
the estimation of the black hole mass using virial estimators, and warns about
such effects for several other quasars characterized by similar line
asymmetries. More generally, our findings may suggest a broader revision of the
decomposition and interpretation of quasar spectral features, in order to take
into account the presence of potential broad blueshifted Balmer absorption
lines. Our high spatial resolution data also reveals redshifted, dynamically
colder nebular emission lines, likely tracing an inflowing stream.
