A "grave affair" is a serious matter or situation that requires attention, caution, and careful handling. It could be a matter of life and death, a complex legal issue, or a sensitive political matter. Other synonyms for "grave affair" include critical issue, serious concern, a matter of urgency, a delicate situation, a weighty matter, or a pressing matter. These terms are often used to describe situations that demand immediate attention and require a prudent and thoughtful approach. In any case, a grave affair demands respect and proper handling to ensure the best possible outcome.