E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Insane Root (The).
Hemlock. It is said that those who eat hemlock can see subjective things as objects. Thus, when Banquo had encountered the witches, who vanished as mysteriously as they appeared, he said to Macbeth, Were such things [really] here, . or have we eaten the insane root, that takes the reason prisoner, so that our eyes see things that are not. (Macbeth, i. 3.)
Other plants take the reason prisoner, as the Pruna insana, the Indian nut, Hoary nightshade.