c.1300, quayle, from O.Fr. quaille, perhaps via M.L. quaccula (cf. Prov. calha, It. quaglia, O.Sp. coalla), from a Gmc. source (cf. O.H.G. quahtala "quail," Ger. Wachtel), imitative of the bird's cry. Or the Eng. word may be directly from Gmc. Slang meaning "young attractive woman" first recorded 1859.
"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.