The comparative form of ghastly, meaning more ghastly, horrifying, or terrifying than something else.
From ghastly (adjective) plus the comparative suffix -er to compare two things. Ghastly itself derives from Old English roots meaning to terrify, and adding -er creates the standard comparative form in English.
English comparatives show our need to measure horror—is this more ghastly than that? The suffix -er does the same work in 'ghastlier' as it does in 'bigger,' showing how language treats beauty and horror with the same grammatical tools.
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