unprovoked
[ˌʌnprəˈvoʊkt]
adj.
（尤指暴力言行）未受招惹而自发的，无缘无故的;
例句:
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an act of unprovoked aggression
无端的侵犯行为
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Stephen was stabbed to death in an unprovoked attack nearly five months ago.
斯蒂芬在近5个月前无故遭到袭击，被捅死了。
-
Her angry outburst was totally unprovoked.
她的暴跳如雷完全是无理取闹。
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Unprovoked attacks on immigrants are on the up across Europe.
来自THETIMES
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It looked like it was completely unprovoked.
来自WSJ
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Florida has the most unprovoked shark attacks recorded anywhere in the world.
来自BBC