  We show that Supernova (SN) 2002kg in NGC 2403, initially classified as Type
II-narrow (IIn), has photometric and spectroscopic properties unlike those of
normal SNe. Its behavior, instead, is more typical of highly massive stars
which experience the short-lived luminous blue variable (LBV) phase toward the
end of their lives. The star, in fact, most resembles the LBV S Doradus in
outburst. The precursor of SN 2002kg is the irregular, bright blue variable
star 37 (V37), catalogued by Tammann & Sandage in 1968. Using high-quality
ground-based, multi-band images we can constrain the initial mass of V37 to be
M_ini >~ 40 M_sun. We find that, although the spectra indicate a nitrogen
enhancement, possibly revealing the products of CNO processing by V37 in the
ejecta, the star lacks a substantial LBV nebula. The outburst from SN
2002kg/V37 is not nearly as energetic as the giant eruptions of the eta
Carinae-like variables, such as SN 1954J/V12, also in NGC 2403. SN 2002kg/V37,
however, is among a growing number of ``SN impostors'' exhibiting a broad range
of outburst energetics during a pre-SN phase of massive-star evolution.
