The word 惊涛骇浪 is formed by combining the two phrases 惊涛 (frightening waves) and 骇浪 (terrifying waves), where 惊 and 骇 are both adjectives meaning frightening or terrifying, and 涛 and 浪 are both nouns meaning waves. This pairing of synonymous elements creates a compound idiom that intensifies the description of dangerously large and tumultuous waves.