Quenched disorder simply means that the disorder is explicitly present in the Hamiltonian, typically under the form of random couplings J among the degrees of freedom. For instance, if we consider the famous Edwards-Anderson model $$ H = -\sum_{\langle i, j \rangle} J_{ij} \sigma_i \sigma_j,$$ the disorder is quenched, meaning that the $J$ are constant on the time scale over which the $\sigma$ fluctuate.
For a nice discussion on quenched disorder, which occurs for spin glasses, (and its distinction from annealed disorder), refer to the paper Spin-Glass Theory for Pedestrians by Tommaso Castellani, and Andrea Cavagna.