A　robust　fault-tolerant　motion/force　controller　is　vital　for　a　fully-actuated　unmanned　aerial　vehicle　performing　contact-based　aerial　manipulation　tasks　(e.g.,　window　cleaning,　infrastructure　inspection　and　so　on)　in　the　presence　of　lumped　disturbances　(including　external　disturbances,　model　uncertainties,　and　actuator　faults)　with　unknown　boundaries.　To　address　this　problem,　a　disturbance-observer-based　adaptive　sliding　mode　impedance　controller　is　proposed　in　this　article.　Using　the　impedance　model　as　a　reference　model,　the　sliding　mode　impedance　controller　is　constructed　with　a　modified　adaptive　super-twisting　gain　technique　to　reduce　the　chattering　phenomenon.　To　avoid　overestimation　of　the　unknown　disturbance　bounds,　a　novel　adaptive　third-order　extended　state　observer　is　developed　to　actively　estimate　the　disturbances　and　compensate　for　the　controller　without　any　priori　knowledge　of　the　disturbance　bounds.　Convergence　of　the　proposed　observer　and　stability　of　the　closed-loop　system　are　analyzed　by　the　Lyapunov　method.　The　effects　of　parameters　in　observers　and　controllers　are　presented　under　measurement　noise　through　numerical　simulations.　In　addition,　guidelines　for　parameter　selections　are　also　provided.　The　advantages　and　effectiveness　of　the　proposed　control　strategy　are　also　demonstrated　by　simulating　a　push-and-slide　scenario　in　the　presence　of　lumped　disturbances.