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1844, American English, short for chaparejos, from Mexican Spanish chaparreras, overalls worn to protect from chaparro (see chaparral).
"jaws, cheeks," from chap (n.), 1550s, of unknown origin. Hence, chap-fallen (1590s).
1570s, "customer," short for obsolete chapman "purchaser, trader" (see cheap). Colloquial sense of "lad, fellow" is first attested 1716 (cf. slang tough customer).
"to crack," mid-15c., chappen (intransitive) "to split, burst open;" "cause to crack" (transitive); perhaps a variant of choppen (see chop (v.), and cf. strap/strop), or related to Middle Dutch kappen "to chop, cut," Danish kappe, Swedish kappa "to cut." Related: Chapped; chapping. The noun meaning "fissure in the skin" is from late 14c.