E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Herb of Grace.
Rue is so called because of its use in exorcism, and hence the Roman Catholics sprinkle holy water with a bunch of rue. It was for centuries supposed to prevent contagion. Rue is the German raute; Greek, rut; Latin, ruta, meaning the preserver, being a preservative of health (Greek, ruo, to preserve). Ophelia calls it the Herb of Grace o Sundays.