The word 底下 is formed by combining two characters that each convey a spatial concept of lower position: 底 (dǐ), meaning "bottom" or "base", and 下 (xià), meaning "below" or "under". Their formation is a straightforward semantic fusion, where the meaning of the second character (下) specifies and reinforces the directional aspect of the first (底), together creating a single, more concrete idea of "the space directly beneath" or "underneath".