We propose a many-body quantum-mechanical model for multiple thermal scattering of fast electrons due to phonon excitation in condensed matter. The key approximation upon which our approach is based is in the spirit of the Born-Oppenheimer approximation. We compare the conceptual underpinnings of this model to the well-known frozen phonon model for thermal scattering. Using our model it is possible to calculate the probability distribution of both elastically and inelastically scattered electrons. The model is also extended to encompass core-shell ionization.