Sad, unhappy. The term refers to a mournful facial expression, with the corners of the mouth drawn down. Known by the mid-seventeenth century, it appears in print in Bishop Joseph Hall’s Cases of Conscience (1649): “The Roman Orator was down in the mouth, finding himselfs thus cheated by the moneychanger.” Occasionally it appeared with at instead of in (“He’ll never more be down-at-mouth,” Dante Gabriel Rossetti, Dante and His Circle, 1850), a usage that is now obsolete. See also down in the dumps.