Quiz: Remember the definition of mal de mer?
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.