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あいつ にしちゃ 思い切った な。
I had a hard time to translate this short sentence. And after I looked around from many sources, I found these clues.
思い切った
普通では考えられないような。おどろくほど大胆な。 「 －処置を取る」
source: https://www.weblio.jp/
にしては (= ni shitewa)
(casual contraction) にしちゃ (= ni shicha)
source: Grammar Lesson from Maggie Sensei's site
あいつ にしちゃ 思い切った な。
My translation:
It's surprisingly bold for (that guy / he / she).
Did I translate the sentence properly, yet?
「あいつ にしちゃ 思{おも}い切{き}った な。」
Without any context -- without even another word or any explanation of the context -- native speakers will know two things for certain from this short sentence.
1) This dude （あいつ） made a big decision and executed it.
We know that because 「思い切る」 as a verb means "to get up the nerve to do something drastic/unusual".
The fact that you, for some reason, looked up the meaning of 「思い切った」 as an adjectival phrase ended up affecting your translation attempt because that forced you to deal with a verb-less sentence.
2) The dude is not the kind of person who is known to take such (drastic) actions.
We know this from the 「あいつにしちゃ」 part, which means "considering the kind of person that he is". 「にしちゃ」 (colloquial form of 「にしては」) has that type of built-in emphasizing factor.
My own TL:
"For/Considering the kind of person that he is, he has done something very drastic."
I think your translation is close. But we can describe it in detail. From your comment,
The sentence was from a short novel, about the intrigue between schoolmates to get rid of each other. In short, the person mentioned in the sentence took the risk. I was just a bit confused since the word can also be translated as "losing one's mind"
From their judgement, You might be guessing the outcome could be going towards wrong direction and you might feel as if one is losing his/her mind. But the expression 「思い切った」focuses on "distinct and unexpected" decision by him/her rather than his/her behavior. Now, I have applied it to the situation you described.
So, the schoolmates secretly having scheme in order not to be annoyed by the opponent. Probably one dobs the opponent in about his/her pranks to the teacher who teaches the subject which the opponent is not good at in order that the opponent is called on at class and being humiliated by not answering the teacher's question.
But this time, the one having been dobbed in as usual is retaliating immediately to the rival regardless of considering the chance of getting further revenge from the rival.
From other people observing, the one doing so was unexpected. So they are commenting like「あいつ にしちゃ 思い切った な。」: "It is bold/daring and unhesitant for him/her to do so."
