The word "刨根问底" is formed by combining two verb-object phrases in a parallel structure: "刨根" (dig roots) and "问底" (ask about the bottom). Here, 刨 (dig) and 问 (ask) are verbs acting on their objects 根 (root) and 底 (base) respectively, resulting in a literal meaning of digging to the roots and inquiring to the bottom, which together express the idea of thorough examination.