means To shrink back, flinch, or lose courage in the face of something frightening — or, as a noun, a small plump ground-dwelling game bird.

from Two threads tangle here. The bird comes from Middle English 'quaille,' borrowed from Old French 'quaille,' likely from Medieval Latin 'quaccula' — an imitation of the bird's clucking call, the same way 'cuckoo' apes its own song. The verb 'to quail,' meaning to cower, has a murkier root: possibly from Middle Dutch 'quelen' (to suffer, be ill) or related to Old English 'cwelan' (to die). The two words almost certainly arrived by separate paths and only later sat side by side in English, so the trembling bird and the trembling coward are coincidental neighbors, not blood relatives.