A more thorough explanation:

A quasi contract is a legal obligation that is imposed by law to prevent unjust enrichment. It is also known as a contract implied in law or a constructive contract. A quasi contract may be presumed by a court in the absence of a true contract, but not where a contract covering the same subject matter already exists.

A quasi contract does not require mutual assent, and a court may impose an obligation without regard to the intent of the parties. When a party sues for damages under a quasi-contract, the remedy is typically restitution or recovery under a theory of quantum meruit. Liability is determined on a case-by-case basis.

For example, if a contractor mistakenly builds a fence on his neighbor's property, the neighbor may be enriched by the new fence. In this case, the contractor may be entitled to compensation for the value of the fence under a quasi contract.

Another example is when a person receives medical treatment without agreeing to pay for it. If the person benefits from the treatment and it would be unfair to not pay for it, a court may impose an obligation to pay under a quasi contract.