Most prone to causing or marked by headaches; the superlative form indicating the greatest degree of head pain or troublesomeness.
From 'headache' with superlative suffix '-est'. This follows standard English patterns for adjectives derived from nouns describing physical conditions.
English lets us turn almost any noun into an adjective by adding '-y', then compare them with '-er' and '-est'—that's why we can have a 'headachier' day than yesterday!
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