  We characterize the LHS 1610 system, a nearby ($d=9.7$ pc) M5 dwarf hosting a
brown dwarf in a $10.6$ day, eccentric ($e \sim 0.37$) orbit. A joint fit of
the available Gaia two-body solution, discovery radial velocities (RVs) from
TRES, and new RVs obtained with the Habitable-zone Planet Finder, yields an
orbital inclination of $117.2\pm0.9^\circ$ and a mass constraint of
$50.9\pm0.9$ M$_J$. This gives LHS 1610 b the second most precise mass of brown
dwarfs orbiting M stars within 25pc. We highlight a discrepancy between the
Gaia two-body solution eccentricity ($e=0.52 \pm 0.03$) and that from the RVs
($e=0.3702\pm0.0003$), which requires the astrometric time-series release (Gaia
DR4) for further diagnostics. With a flare rate of $0.28\pm 0.07$ flares/day
from TESS photometry, and a rotation period of $84 \pm 8$ days, LHS 1610 joins
other mid M stars -- including Proxima Centauri and YZ Ceti -- as nearby mid M
dwarfs with flare rates on the higher end for their long rotation periods.
These stars are promising candidates for searching for sub-Alfv\'enic
star-companion interactions, raising the question whether LHS 1610 b could be
driving the flares on its host star. However, the available TESS photometry is
insufficient to confirm or rule out any orbital phase-dependence of the flares.
We show that the LHS 1610 system, as a nearby mid M star with a large,
short-period companion, is a promising target to look for evidence of
star-companion interactions or aural emission from the brown dwarf at radio
wavelengths.
