  We report the discovery of a high velocity, very low-mass star or brown dwarf
whose kinematics suggest it is unbound to the Milky Way. CWISE
J124909.08+362116.0 was identified by citizen scientists in the Backyard
Worlds: Planet 9 program as a high proper motion ($\mu$ $=$ 0''9/yr) faint red
source. Moderate resolution spectroscopy with Keck/NIRES reveals it to be a
metal-poor early L subdwarf with a large radial velocity ($-$103$\pm$10 km/s),
and its estimated distance of 125$\pm$8 pc yields a speed of 456$\pm$27 km/s in
the Galactic rest frame, near the local escape velocity for the Milky Way. We
explore several potential scenarios for the origin of this source, including
ejection from the Galactic center $\gtrsim$3 Gyr in the past, survival as the
mass donor companion to an exploded white dwarf. acceleration through a
three-body interaction with a black hole binary in a globular cluster, and
accretion from a Milky Way satellite system. CWISE J1249+3621 is the first
hypervelocity very low mass star or brown dwarf to be found, and the nearest of
all such systems. It may represent a broader population of very high velocity,
low-mass objects that have undergone extreme accelerations.
