  Nearby M dwarf systems currently offer the most favorable opportunities for
spectroscopic investigations of terrestrial exoplanet atmospheres. The LTT~1445
system is a hierarchical triple of M dwarfs with two known planets orbiting the
primary star, LTT~1445A. We observe four transits of the terrestrial world
LTT~1445Ab ($R=1.3$ R$_\oplus$, $M=2.9$ M$_\oplus$) at low resolution with
Magellan II/LDSS3C. We use the combined flux of the LTT~1445BC pair as a
comparison star, marking the first time that an M dwarf is used to remove
telluric variability from time-series observations of another M dwarf. We find
H$\alpha$ in emission from both LTT~1445B and C, as well as a flare in one of
the data sets from LTT~1445C. These contaminated data are removed from the
analysis. We construct a broadband transit light curve of LTT~1445Ab from
620--1020 nm. Binned to 3-minute time bins we achieve an rms of 49 ppm for the
combined broadband light curve. We construct a transmission spectrum with 20
spectrophotometric bins each spanning 20 nm and compare it to models of clear,
1$\times$ solar composition atmospheres. We rule out this atmospheric case with
a surface pressure of 10 bars to $3.2 \sigma$ confidence, and with a surface
pressure of 1 bar to $3.1 \sigma$ confidence. Upcoming secondary eclipse
observations of LTT~1445Ab with JWST will further probe the cases of a high
mean molecular weight atmosphere, a hazy or cloudy atmosphere, or no atmosphere
at all on this terrestrial world.
