Thermoelectric efficiency benefits from suppressing lattice thermal conductivity (${\kappa }_l$), yet the microscopic origin of the glasslike ultralow ${\kappa }_l$ in ${\text{AgSbTe}}_2$ remains debated. Here we combine first-principles simulation of anharmonic lattice dynamics with the unified theory of thermal transport to quantify how Ag-Sb cation disorder controls heat conduction. We find that compared with cation-ordered $\mathrm{L}{1}_1$ and D4 phases, disordered structures exhibit a strongly reduced ${\kappa }_l$ across a wide temperature range [approximately equal to $0.48\text{W}/(\mathrm{m}\mathrm{K})$ at 300 K] and a much weaker temperature dependence ($\propto T^{-0.40}$). Our detailed analysis of the phonon spectrum and mode-resolved transport properties reveals that cation disorder disrupts the Te-centered octahedral network, softens phonons, lowers group velocities, and enhances low-frequency scattering, thus dramatically suppressing the diagonal (particlelike) contribution [approximately equal to $0.07\mathrm{W}/(\mathrm{m}\mathrm{K})$] while making the off-diagonal (wavelike) term dominant. Moreover, we performed a statistical analysis of ${\kappa }_l$ of 434 dynamically stable cation-disordered configurations, which reveals a robust positive correlation between ${\kappa }_l$ and the average frequency of acoustic modes. In addition, acoustic softening further correlates with increased Ag and Te participation and reduced participation ratios, indicating disorder-induced localization of heat-carrying modes. These results identify cation-disorder-driven localization and low-frequency scattering as key to the phonon-glass behavior of ${\text{AgSbTe}}_2$, and suggest disorder engineering as a route to tune ${\kappa }_l$ in multication thermoelectrics.

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• Anharmonic lattice dynamics
• Lattice thermal conductivity
• Thermoelectrics
• Disordered systems
• First-principles calculations