The interplay between structure distortion and lattice dynamics provides a compelling strategy to suppress phonon transport in thermoelectric materials. In this work, we explore the origin of highly anisotropic thermal conductivity in ${\mathrm{Sn}}_2{\mathrm{S}}_3$ through first-principles calculations combined with the Wigner transport equation (WTE). We find that the stereochemically active lone-pair electrons of ${\mathrm{Sn}}^{2+}$ pronounced local structural distortions, characterized by asymmetric Sn–S bond angles and weakened bonding configurations. These distortions activate rattling modes, which flatten vibrational branches, enhance phonon scattering, and reduce group velocities, particularly along the $x$ and $z$ directions. In contrast, the $y$ direction supports more coherent and dispersive phonon transport, leading to markedly higher thermal conductivity. Additionally, the influence of rattling extends to coherent phonon transport, yielding a complex frequency-dependent interplay between particlelike and wavelike heat conduction. Our results establish a direct link between lone-pair-induced anharmonicity and anisotropic phonon transport, offering a design framework for low-$\kappa$ materials beyond van der Waals or interlayer-driven strategies.

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• Phonons
• Thermal conductivity
• Thermal properties
• Boltzmann theory
• First-principles calculations