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To vitiate something is to spoil it, it, or ruin how well it works — usually not by smashing it, but by letting something bad get in so the thing is no longer sound. A little poison in a well, one lie in a long report: the whole thing is now vitiated. There are two places you meet it. The first is general: fumes vitiate the air; bias vitiates a study; one careless mistake vitiates all the careful work around it. Notice that the subject is usually the bad thing, not a person — 'smoke vitiated the air', not 'he vitiated the air'. The second is legal, and it is the one you will actually see most often. In law, to vitiate a contract, a will, or an agreement means to take away its legal force — to make it no longer count. Lawyers say 'fraud vitiates the contract', meaning: because there was fraud, the contract is dead. In this legal use the meaning is close to 'cancel' or 'make invalid', and it is stronger than just ''. The feel of the word is formal and rather old — you find it in law reports, philosophy, and serious writing, almost never in speech. If a friend told you the rain vitiated their picnic, they would be joking. The usual quiet partner in normal English is 'spoil', 'undermine', or 'invalidate'. Note also that it is quite often used in the passive: 'the results were vitiated by a faulty machine'.
損害、使無效；（法律）使（契約等）失效
組成From Latin vitium 'a fault, a flaw, a vice' — so vitiate literally means 'to put a fault into'. The verb vitiō meant 'to damage, to spoil'; the -ate ending is the ordinary English verb ending built from Latin past participles (like activate, create).
原始詞典資料4 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
vitiateverb/ˈvɪʃ.i.eɪt/
- 1
To spoil, make faulty; to reduce the value, quality, or effectiveness of something.
- The least admixture of a lie, -- for example, the taint of vanity, the least attempt to make a good impression, a favorable appearance, -- will instantly vitiate the effect.
- Such diversion as Podson could extort from his isolation was soon vitiated by repetition. He surfed. He sun-baked - with discretion till his skin had peeled and given him a harder cuticle.
- 2
To debase or morally corrupt.
- Dissolute and vitiated alike, they confided in, and ever acted in mutual concert with each other's plans, according to the deep subtleties of their reasonings, which linked them together by some secret spell.
- There was excellent blood in his veins—royal stuff; though sadly vitiated, I fear, by the cannibal propensity he nourished in his untutored youth.
- 3
archaicTo violate, to rape.
- ‘Crush the cockatrice,’ he groaned, from his death-cell. ‘I am dead in law’ – but of the girl he denied that he had ‘attempted to vitiate her at Nine years old’; for ‘upon the word of a dying man, both her Eyes did see, and her Hands did act in all that was done’.
- 4
To make something ineffective, to invalidate.
- […]all the hinges of the animal frame are subverted, every animal function is vitiated; the carcass retains but just life enough to make it capable of suffering.
- He went over his canvases with disgust and anger, unable to see virtue in any one of them. Even his sacred Oyster Girl went back on him. The creature of a vitiated æstheticism, he could only suppose that conceit had played an abominable trick on his eyesight.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。