Comparative form of achy, meaning more painful or having more aches than something else.
From achy (adjective from ache) plus the comparative suffix -er. Ache comes from Old English æce, related to Gothic agis and having roots in Proto-Germanic pain-related words.
The progression 'achy → achier → achiest' shows how English handles comparison with one-syllable words ending in 'e'—we drop the e before adding -er, a rule that seems simple until you realize how many exceptions English has!
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