The behaviour of a fluid at the interface with a solid boundary is affected, to a large extent, by the potential landscape imposed on the fluid by the solid. Fluid slip at the interface with a solid boundary is modelled here as forced Brownian motion in a periodic potential landscape. The resulting model goes beyond simple transition-state-theory approaches and uses well-defined atomistic parameters to capture the salient features of the slip process in both the linear and nonlinear forcing regimes, yielding excellent agreement with molecular dynamics simulation results, as well as previous modelling results. An explicit expression for the Navier slip coefficient in terms of molecular-level system parameters is derived.