"to lose heart, to shrink," mid-15c., of unknown origin, perhaps from M.Du. quelen "to suffer, be ill," from P.Gmc. *kwel- "to die" (see
quell). Or from obsolete quail "to curdle" (late 14c.), from O.Fr. coailler, from L. coagulare (see
coagulate). Sense of "cower" is attested from 1550s. Common 1520-1650, then rare until 19c.; apparently revived by Scott.