Reducing thermal conductivity (κ) is of great significance to lots of applications, such as thermal insulation, thermoelectrics, etc. In this study, we propose an effective approach for realizing low κ by introducing lone-pair electrons or making the lone-pair electrons stereochemically active through bond nanodesigning. By cutting at the (111) cross section of the three-dimensional cubic boron arsenide (c-BAs), the κ is lowered by more than 1 order of magnitude in the resultant two-dimensional graphene-like BAs (g-BAs). The underlying mechanism of activating lone-pair electrons is analyzed based on the comparative study on the thermal transport properties and electronic structures of g-BAs, c-BAs, graphene, and diamond (c-BAs → g-BAs vs diamond → graphene). The proposed approach for realizing low κ and the underlying mechanism uncovered in this study would largely benefit the design of advanced thermal functional materials, especially in future research involving novel materials for energy applications.