E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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
Theres rue for you, and heres some for me! we may call it herb of grace o Sundays.Shakespeare: Hamlet, iv. 5.