The word 噩梦 is formed by directly combining two meaningful characters: 恶 (è), which conveys a sense of badness, evil, or something terrible, and 梦 (mèng), which means "dream". They are placed together in a straightforward, conceptual union where the first character modifies the second, creating the composite idea of a "bad dream" or "nightmare". It is a simple semantic fusion where one concept directly describes the nature of the other.