  We present the discovery of NGTS-7Ab, a high mass brown dwarf transiting an M
dwarf with a period of 16.2 hours, discovered as part of the Next Generation
Transit Survey (NGTS). This is the shortest period transiting brown dwarf
around a main or pre-main sequence star to date. The M star host (NGTS-7A) has
an age of roughly 55 Myr and is in a state of spin-orbit synchronisation, which
we attribute to tidal interaction with the brown dwarf acting to spin up the
star. The host star is magnetically active and shows multiple flares across the
NGTS and follow up lightcurves, which we use to probe the flare-starspot phase
relation. The host star also has an M star companion at a separation of 1.13
arcseconds with very similar proper motion and systemic velocity, suggesting
the NGTS-7 system is a hierarchical triple. The combination of tidal
synchronisation and magnetic braking is expected to drive ongoing decay of the
brown dwarf orbit, with a remaining lifetime of only 5-10 Myr.
