A more thorough explanation:

An offensive-use waiver is a legal term that refers to an exemption from the attorney-client privilege. This means that a litigant (someone involved in a lawsuit) may be considered to have waived their right to keep certain information confidential if they seek affirmative relief (a legal remedy) and their claim relies on privileged information that would be outcome-determinative (meaning it would significantly impact the outcome of the case) and the opposing party has no other way to obtain that information.

For example, let's say that a company is involved in a lawsuit with a former employee who is claiming that they were wrongfully terminated. The company's defense relies on privileged information that was discussed between the company and their legal counsel. If the former employee has no other way to obtain that information and the company seeks affirmative relief (such as asking the court to dismiss the case), they may be considered to have waived their right to keep that information confidential.

Another example could be a criminal case where the defendant's defense relies on privileged information discussed between them and their attorney. If the prosecution has no other way to obtain that information and the defendant seeks affirmative relief (such as asking the court to dismiss the charges), they may be considered to have waived their right to keep that information confidential.