The word '眼线' is formed by directly combining the character for 'eye' (眼) and the character for 'line' (线), resulting in the literal meaning 'eye line.' This construction yields the term for eyeliner in cosmetics, where a line is drawn around the eye. It also figuratively extends to mean an informant or lookout, with 'eye' implying surveillance and 'line' implying a connection or channel of information.