c. 1400, "have a morbid craving;" early 15c., "grow feeble or sick;" mid-15c., "to fade, fail, give way," of unknown origin, perhaps from Middle Dutch quelen "to suffer, be ill," from Proto-Germanic *kwaljan, from PIE root *gwele- "to throw, reach," with extended sense "to pierce."
Or from obsolete quail "to curdle" (late 14c.), from Old French coailler, from Latin coagulare (see coagulate). Sense of "lose heart, shrink, cower" is attested from 1550s. According to OED, common 1520-1650, then rare until 19c., when apparently it was revived by Scott. Related: Quailed; quailing.