  CGRaBS J0809+5341, a high redshift blazar at z = 2.144, underwent a giant
optical outburst on 2014 April 19 when it brightened by $\sim$5 mag and reached
an unfiltered apparent magnitude of 15.7 mag. This implies an absolute
magnitude of -30.5 mag, making it one of the brightest quasars in the Universe.
This optical flaring triggered us to carry out observations during the decaying
part of the flare covering a wide energy range using the {\it Nuclear
Spectroscopic Telescope Array}, {\it Swift}, and ground based optical
facilities. For the first time, the source is detected in $\gamma$-rays by the
Large Area Telescope onboard the {\it Fermi Gamma-Ray Space Telescope}. A high
optical polarization of $\sim$10\% is also observed. Using the Sloan Digital
Sky Survey spectrum, accretion disk luminosity and black hole mass are
estimated as $1.5 \times 10^{45}$ erg s$^{-1}$ and $10^{8.4}~M_{\odot}$
respectively. Using a single zone leptonic emission model, we reproduce the
spectral energy distribution of the source during the flaring activity. This
analysis suggests that the emission region is probably located outside the
broad line region, and the jet becomes radiatively efficient. We also show that
the overall properties of CGRaBS J0809+5341 seems not to be in agreement with
the general properties observed in high redshift blazars up to now.
