What is the test for unconscionable dealing?

38. One of the oldest descriptions of the doctrine of unconscionability is found in Slator v. Nolan, Ir. R. 11 Eq. 386: If two persons, no matter whether a confidential relationship exists between them or not, stand in such a relation to each other that one can take an undue advantage of the other, whether by reason of distress, or recklessness or wildness, or want of care, and when the facts shew that one party has taken undue advantage of the other by reason of the circumstances I have mentioned, a transaction resting upon such unconscionable dealing will not be allowed to stand; and there are several cases which shew,1 even where no confidential relationship exists, that where parties were not on equal terms, the party who gets a benefit cannot hold it without proving that everything has been right and fair and reasonable on his part.