In an archaic or rare sense, meaning in a manner that is full of horror, terror, or ghastliness.
From ghastful plus the suffix -ly to form an adverb. This is a doubly archaic formation (archaic adjective + adverb suffix), making it quite rare in historical texts and essentially extinct in modern usage.
This word is so archaic it's practically a linguistic fossil—it shows how English speakers of centuries past built adverbs methodically by adding -ly to any adjective, even ones we'd no longer use today.
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