  We present photometric and spectroscopic observations of SN Hunt 248, a new
supernova (SN) impostor in NGC 5806, which began a multi-stage outburst in 2014
May. The initial '2014a' discovery brightening exhibited an absolute magnitude
of M~-12 and the spectral characteristics of a cool dense outflow, including
P-Cygni lines of Fe II, H I, Na I, and strong line blanketing from metals. The
source rapidly climbed and peaked near M~-15 mag after two additional weeks.
During this bright '2014b' phase the spectrum became dominated by Balmer
emission and a stronger blue continuum, similar to the SN impostor SN 1997bs.
Archival images from the Hubble Space Telescope between 1997 and 2005 reveal a
luminous (4e5 Lsun) variable precursor star. Its location on the
Hertzsprung-Russell diagram is consistent with a massive (M_init~30 Msun) cool
hypergiant having an extremely dense wind and an Eddington ratio just below
unity. At the onset of the recent 2014a brightening, however, the object became
super-Eddington. The subsequent boost in luminosity during the 2014b phase
probably resulted from circumstellar interaction. SN Hunt 248 provides the
first case of a cool hypergiant undergoing a giant eruption reminiscent of
outbursts from luminous blue variable (LBV) stars. This lends support to the
hypothesis that some cool hypergiants, such as Rho Cas, could be LBVs
masquerading under a pseudo-photosphere created by their extremely dense winds.
Moreover, SN Hunt 248 demonstrates that eruptions stemming from such stars can
rival in peak luminosity the giant outbursts of much more massive systems like
Eta Car.
