The word "火线" is formed by directly combining the character "火" (fire), which here conveys concepts of danger, intensity, or active energy, with the character "线" (line or wire), denoting a physical or metaphorical line. This creates a compound where the first character modifies the second, specifically indicating a line that is electrically live or a military frontline, thus deriving its meaning from the attributes of "fire" applied to a "line".