  We recently reported the discovery of FORJ0332-3557, a lensed Lyman-break
galaxy at z=3.77 in a remarkable example of strong galaxy-galaxy gravitational
lensing. We present here a medium-resolution rest-frame UV spectrum of the
source, which appears to be similar to the well-known Lyman-break galaxy
MS1512-cB58 at z=2.73. The spectral energy distribution is consistent with a
stellar population of less than 30 Ma, with an extinction of A(V)=0.5 mag and
an extinction-corrected star formation rate SFR(UV) of 200-300 Msun/a. The
Lyman-alpha line exhibits a damped profile in absorption produced by a column
density of about N(HI) = (2.5+_1.0) 10^21 atoms/cm^2, superimposed on an
emission line shifted both spatially (0.5 arcsec with respect to the UV
continuum source) and in velocity space (+830 km/s with respect to the
low-ionisation absorption lines from its interstellar medium), a clear
signature of outflows with an expansion velocity of about 270 km/s. A strong
emission line from HeII 164.04nm indicates the presence of Wolf-Rayet stars and
reinforces the interpretation of a very young starburst. The metallic lines
indicate sub-solar abundances of elements Si, Al, and C in the ionised gas
phase.
