This letter addresses the robust trajectory tracking problem for a Quadrotor-Slung-Load System (QSLS), which consists of a point-mass load and a rigid-body quadrotor connected by an inelastic cable. To construct the controller, we employ the backstepping technique and propose an Uncertainty and Disturbance Estimator (UDE) to compensate for uncertainties arising from imprecise model parameters and exogenous time-varying disturbances affecting both quadrotor and load. The main feature of the UDE is its ability to convert the robust control problem into a low-pass filter design in the frequency domain, which generates an estimate of lumped uncertainties. To streamline the design process, we utilize a coordinate transformation strategy that converts the QSLS into a configuration that resembles the dynamics of a typical quadrotor system. The proposed controller ensures uniformly ultimate boundedness of closed-loop errors in the presence of time-varying exogenous disturbances, while guaranteeing asymptotic stability when disturbances are zero. Finally, we present comprehensive simulation and experimental results to validate the effectiveness and robustness of the proposed solution.