The word "垂涎三尺" is formed by directly combining the character 垂 (to droop) with 涎 (saliva) to depict drooling, followed by 三 (three) and 尺 (a foot, unit of length) to exaggerate the length of the drool, thus creating a vivid, literal image that metaphorically conveys extreme craving through this hyperbolic physical description.