To gainsay something means to deny, contradict, or declare it false. When you gainsay a claim, you're saying it isn't true or shouldn't be accepted. If your friend insists that the school cafeteria serves the best pizza in town, you might gainsay that statement by pointing out three better pizza places within walking distance.

The word carries a formal, somewhat old-fashioned tone. You're more likely to encounter it in books than in everyday conversation. A lawyer might argue that the evidence cannot be gainsaid, meaning it's so strong that no one can reasonably dispute it. A historian might write that Napoleon's military genius is not to be gainsaid, meaning it's undeniable despite his ultimate defeat.

Gainsay is stronger than simply disagreeing. When you gainsay something, you're asserting that the original statement is wrong or false, not just offering a different opinion. If someone says “There's no gainsaying the facts,” they mean the facts speak for themselves and attempting to contradict them would be pointless or dishonest.

The word is often used in negative constructions: “no one can gainsay,” “not to be gainsaid,” or “cannot be gainsaid,” which emphasize that something is too obviously true to argue against.