Forewarned is forearmed 是個諺語，意為「先知先防備；有備無患」。這個諺語於 1500 年代開始使用，用來談論軍隊要事先做好防備，有備則無患；但它不久就被用在其他情況，而不限於軍事。大多數人都認為這個諺語源自拉丁文一個意思相當的片語 “Praemonitus, praemunitus”。
例句：
- Jack says that Monica is very angry with me still. Well, forewarned is forearmed, and I’ll have to think up a good excuse before I see her. (傑克說，莫妮卡仍很生我的氣。所以，有備無患，我必須在見她之前想出一個好的藉口)
- Before you meet Sarah, I should tell you that she’s a little eccentric. Forewarned is forearmed, right? (在你跟莎拉見面之前，我應該告訴你，她有點古怪。先知先防備，對不對？)
- Hey, just so you know, the boss is in a really foul mood today. Forewarned is forearmed, right? (嗨，只是讓你知道一下，老闆今天的心情很差。先知先防備，對不對？)