A more thorough explanation:

Definition: Lesio enormis is a legal term that refers to an excessive loss or abnormal loss of more than half. It was used in Roman and civil law to describe two situations:

1. The sale of a thing for which the buyer paid less than half its real value. In this case, the seller could rescind the sale, but the buyer could keep the item purchased by paying the full value. This doctrine was usually applied to land sales.
2. The injury sustained by one party to an onerous contract when the overreaching party receives twice the value of that party's 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, laesio enormis may exist. If co-owner co-heirs partition or sell property, laesio enormis may exist when the purchaser pays less than one-fourth of the value rather than one-half.

For instance, if a person buys a house for $50,000, but its real value is $100,000, the seller can rescind the sale, and the buyer can keep the house by paying the full value of $100,000. Similarly, if a seller sells a property for $200,000, but its real value is $100,000, the purchaser may have suffered laesio enormis, and the seller may have to rescind the sale or pay the difference.

Lesio enormis was a rule established very late, and it became the means of testing the validity of contracts generally by their fairness. It is a principle embodied in the German Civil Code and the Swiss Code of Obligations. In modern courts, it invests judges with discretion not very likely to be abused, but sufficient to act as a deterrent to the grosser forms of economic exploitation.