E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Rue,
called “herb of grace,” because it was employed for sprinkling holy water. Without doubt it was so used symbolically, because to rue means to be sorry, and penitence brings the water of grace with it. (Latin, ruta, from the Greek rhuo, so called because it sets persons free from disease and death.) (See DIFFERENCE.) Ophelia says—
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“There’s rue for you, and here’s some for me! we may call it herb of grace’ o’ Sundays.”—Shakespeare: Hamlet, iv. 5.
