1570s, "customer," short for obsolete chapman "purchaser, trader" (see cheap). Colloquial sense of "lad, fellow" is first attested 1716 (compare 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 compare 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.