The process of skin becoming cracked and sore, usually from cold, wind, or dryness; or the present participle of the verb 'chap.'
From Middle English 'chappen,' possibly from Old Norse 'kappa' (to strike or cut). The meaning evolved from cutting/striking to describe how weather 'strikes' and damages skin.
Chapped lips become painful because the skin there lacks oil glands—it's literally the thinnest, most vulnerable skin on your entire body, which is why medieval people used beeswax balms centuries before modern lip balms.
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