A more thorough explanation:

A case of first impression is a legal case that presents a new legal issue that has never been decided before by the governing jurisdiction. This means that there is no existing legal precedent or prior court decision that can be used to guide the court's decision.

For example, the 1978 Supreme Court case Monell v. Department of Soc. Svcs. was a case of first impression because it was the first time the court had to decide whether local governments were considered "persons" under the Civil Rights Act of 1871. Since there was no prior court decision on this issue, the court had to rely on other sources to make its decision.

When deciding a case of first impression, courts will look to various sources for guidance, such as legislative history and intent, policy, custom, the Restatement view, and the law in other jurisdictions. This helps the court to adopt the most persuasive rule of law.

Overall, a case of first impression is an important legal concept because it allows courts to make decisions on new legal issues that have not been addressed before, and helps to shape the development of the law.