Английский Этимологический словарь - quail (v.)

- c.1400, from M.Du. quelen "to suffer, be ill," from P.Gmc. *kwel- "to die." Or from obsolete quail "to curdle," from O.Fr. coailler, from L. coagulare (see coagulate). Sense of "cower" is from 1555. Common 1520-1650, then rare until 19c., apparently revived by Scott.