1. Fatal, deadly, lethal, mortal apply to something that has caused or is capable of causing death. Fatal may refer to either the future or the past; in either case, it emphasizes inevitability and the inescapable—the disastrous, whether death or dire misfortune: The accident was fatal. Such a mistake would be fatal. Deadly looks to the future, and suggests that which is likely to cause death (though not inevitably so): a deadly poison, disease. Like deadly, lethal looks to the future but, like many other words of Latin origin, suggests a more technical usage: a lethal dose; a gas that is lethal. Mortal looks to the past and refers to death that has actually occurred: He received a mortal wound. The disease proved to be mortal. 2. ruinous, disastrous, calamitous, catastrophic, devastating. 4. predestined, foreordained.