We present a theory of orientation Brownian motion of a polar particle frozen-in in a polymer glass. The theoretical problem is then one of orientation dynamics in a viscoelastic medium. The glassy material provides, at a short-time scale, an elastic force that returns the orientation to that at time t=0. On larger time scales, this privileged orientation diffuses due to thermal motion. A cascade of orientation motions at all time scales results in a stretched-exponential behavior of the nonlinear optical response function and of other moments of the angular distribution function.