The phrase "inform against" typically means to report someone's wrongdoing or criminal behavior to authorities. However, there are a variety of synonyms that can be used to convey similar meanings. A few examples include: snitch on, rat out, report, betray, accuse, denounce, and expose. Each of these words emphasizes different aspects of informing against someone, whether it be the act of reporting, the betrayal of trust, or the accusation of wrongdoing. It is important to choose the appropriate synonym for the specific context to accurately convey the meaning and intention behind the informing against someone.