The word "蛤蟆镜" is formed by combining the noun "蛤蟆" (toad) with "镜" (glasses), where "蛤蟆" acts as a descriptive modifier based on visual metaphor. It specifically refers to a style of sunglasses with large, circular lenses that are thought to resemble the bulging eyes of a toad, thus creating a compound term that denotes these glasses through direct analogy.