The quality or state of having aches; persistent dull pain throughout the body.
From 'achy' (adjective, from Old English 'acan' meaning to ache) + suffix '-ness' (forming nouns of quality). The combination became common in American English in the early 1900s.
Interestingly, 'achiness' became a medical term doctors specifically use for viral infections—it's so common after flu that the word describes a specific type of pain doctors instantly recognize!
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