  We report here a giant zero-point energy renormalization of 273 meV in the
direct band-gap at $\textbf{K}$ and a 571 meV of blue-shifting in the position
of the doubly-degenerate brightest excitonic peak in monolayer hexagonal boron
nitride. The non-radiative exciton linewidth is found to be 97 meV at 0K with a
large coupling strength of 1.1 eV. This linewidth is found to be mainly
dominated by the scattering from the longitudinal optical phonons near the
degenerate LO-TO mode, with negligible contributions from other lower branches.
Additionally, the band-gap has a temperature dependent slope of -0.53
meVK$^{-1}$, which we found to be in excellent agreement with the reported
experimental data on large diameter boron nitride nanotubes. We obtained our
results by solving a coupled electron-hole Bethe-Salpeter equation including
the lattice vibrational dynamics, purely using ab-initio formalism.
