<p>In this paper, a practical prescribed-time second-order sliding mode (PTSOSM) control scheme is developed for a class of uncertain nonlinear systems subject to deferred asymmetric output constraints. To remove the initial feasibility restriction of conventional output-constrained methods, a composite nonlinear coordinate transformation is introduced to convert the sliding mode dynamics subject to deferred asymmetric output constraints into an equivalent unconstrained system. Based on this transformation, a PTSOSM controller is explicitly constructed using the backstepping technique, and the practical prescribed-time stability of the resulting closed-loop system is established through rigorous Lyapunov analysis. Distinct from existing results, the presented control framework offers two notable advantages: (i) the deferred asymmetric output constraint problem is effectively addressed through the proposed system transformation without imposing an initial feasibility condition; and (ii) the sliding variables achieve practical prescribed-time convergence with a prescribed tracking accuracy, where both the convergence time and steady-state error can be specified a priori, independent of system initial conditions. Finally, numerical and robotic manipulator simulation results are provided to demonstrate the effectiveness of the developed PTSOSM control strategy.</p>