The magnetic properties and crystal electric field (CEF) scheme of ${\mathrm{BiErGeO}}_5$ are investigated via magnetization, heat capacity, inelastic neutron scattering (INS) and muon spin relaxation ($\mu \mathrm{SR}$) experiments on a polycrystalline sample. The ${\mathrm{Er}}^{3+}$ ions form a quasi-two-dimensional distorted honeycomb network with a Kramers doublet ground state. Magnetic susceptibility and heat capacity reveal short-range antiferromagnetic correlations, manifested as a broad maximum around 1.4 K and 1.1 K, respectively. Heat-capacity data further confirm the onset of a magnetic long-range order at $T_{\mathrm{N}}\simeq 0.4\mathrm{K}$. The INS spectra exhibit eight CEF excitations, and the CEF analysis yields the $g$-factor anisotropy with $g_{xy}/g_z\simeq 1.38$ and exchange anisotropy with ${\mathcal{J}}_{xy}\simeq 2.96\mathrm{K}$ and ${\mathcal{J}}_z\simeq 1.56\mathrm{K}$. The experimental temperature and field-dependent magnetization and heat capacity are also reproduced by the simulation using the CEF energy scheme. Zero-field $\mu \mathrm{SR}$ measurements down to 30 mK do not exhibit coherent oscillations or a static $1/3$ tail. The spectra are well described by two exponential relaxation components, indicating two magnetically inequivalent muon environments. The relaxation rates display a nearly temperature-independent plateau below $T_{\mathrm{N}}$ and follow an Orbach-type activated behavior at higher temperatures involving excited CEF levels, consistent with the INS results. Longitudinal-field $\mu \mathrm{SR}$ measurements reveal only weak decoupling up to 1.5 T, indicating the presence of slow spin fluctuations that persist well below $T_{\mathrm{N}}$. Overall, ${\mathrm{BiErGeO}}_5$ emerges as an anisotropic spin-orbit-coupled honeycomb magnet in which strong CEF-driven anisotropy and reduced dimensionality give rise to unconventional low-temperature magnetic behavior distinct from its Yb counterpart ${\mathrm{BiYbGeO}}_5$.

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• Antiferromagnetism
• Crystal-field excitations
• Spin dynamics
• Spin-orbit coupling
• Honeycomb lattice
• Inelastic neutron scattering
• Muon spin resonance