A more thorough explanation:

Lesion beyond moiety refers to a legal concept in Roman and civil law where a party suffers an injury in an onerous contract when the other party receives more than double the value of their money or property. For example, if a purchaser pays less than half the value of the property sold, or a seller receives more than double the property's value, lesion beyond moiety may exist.

In the case of a sale, if the buyer paid less than half the real value of the thing sold, the seller could rescind the sale. However, the buyer could keep the item purchased by paying the full value. This doctrine was generally limited to land sales.

For co-owners or co-heirs who partition or sell property, lesion beyond moiety may exist when the purchaser pays less than one-fourth of the value rather than one-half.

Lesion beyond moiety, also known as laesio enormis, was adopted in all modern civilian systems and became the means of testing the validity of contracts generally by their fairness.

For example, if a person sells a house worth $100,000 to another person for only $30,000, the buyer may be able to rescind the sale if lesion beyond moiety is proven. This is because the seller received less than half the real value of the property.