In a ghastly or horrifying manner; in a way that causes terror, dread, or disgust; resembling or suggesting something ghastly.
From ghastly (adjective) plus the adverbial suffix -ly, added to the adjective to modify verbs or other adjectives. Some sources also list this as an alternate form of ghastily, both deriving from the same root.
Ghastlily and ghastily are variant spellings that show how English speakers weren't always sure how to convert ghastly to an adverb—should you double the 's' or not? Different texts chose differently, and eventually ghastly won out as the standard.
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