Ultralow thermal conductivity (κ) underpins thermal insulation and nonequilibrium energy conversion at large temperature gradients. In compositionally pure amorphous silicon with predominantly tetrahedral coordination, a conventional benchmark for minimum thermal conductivity is provided by the Cahill-Watson-Pohl (CWP) model, which constrains the phonon mean free path to half the wavelength. Here, we show an anomalously low κ down to 0.67 W/mK at 300 K, about 33% below this conventional CWP benchmark, in amorphous Si obtained via high-pressure phase engineering. By combining the unified heat-transport framework with molecular dynamics simulations, we attribute the ultralow κ to unusually undercoordinated Si sites generated by rapid mechanical quenching from high pressure. These sites generate inhomogeneous local force fields and seed rattling-like lattice dynamics, leading to strong anharmonicity and vibrational-mode localization. Our results offer a microscopic blueprint to engineer solids with anomalously low κ with undercoordination in amorphous networks without introducing alloying or complex-unit-cell architectures.