The word 底线 (dǐxiàn) is formed through a direct and vivid conceptual combination: 底 means "bottom" or "base", and 线 means "line". Together, they create the mental image of a bottom line, the ultimate, foundational boundary beneath which one cannot or should not go. This inherently spatial and visual metaphor is extended from the literal (like a line on a playing court) to the abstract, representing the fundamental limit of principles, morals, or tolerance, forming a single, cohesive concept.