  We present Hubble Space Telescope (HST) imaging and spectroscopy of the
gravitational lens SL2SJ02176-0513, a cusp arc at z=1.847. The UV continuum of
the lensed galaxy is very blue, which is seemingly at odds with its redder
optical colors. The 3D-HST WFC3/G141 near-infrared spectrum of the lens reveals
the source of this discrepancy to be extremely strong [OIII]5007 and H-beta
emission lines with rest-frame equivalent widths of 2000 +/- 100 and 520 +/- 40
Angstroms, respectively. The source has a stellar mass ~10^8 Msun, sSFR \sim
100/Gyr, and detection of [OIII]4363 yields a metallicity of 12 + log(O/H) =
7.5 +/- 0.2. We identify local blue compact dwarf analogs to SL2SJ02176-0513,
which are among the most metal-poor galaxies in the SDSS. The local analogs
resemble the lensed galaxy in many ways, including UV/optical SED, spatial
morphology and emission line equivalent widths and ratios. Common to
SL2SJ02176-0513 and its local counterparts is an upturn at mid-IR wavelengths
likely arising from hot dust heated by starbursts. The emission lines of
SL2SJ02176-0513 are spatially resolved owing to the combination of the lens and
the high spatial resolution of HST. The lensed galaxy is composed of two clumps
with combined size r_e \sim 300 pc, and we resolve significant differences in
UV color and emission line equivalent width between them. Though it has
characteristics occasionally attributed to active galactic nuclei, we conclude
that SL2SJ02176-0513 is a low-metallicity star-bursting dwarf galaxy. Such
galaxies will be found in significant numbers in the full 3D-HST grism survey.

Cosmology and Nongalactic Astrophysics