Etymology can only ask why a small cuplike guard used in sewing is called a thimble, though it is put on the third finger.

词源学只能问，为什么缝纫时使用的一个小杯状装置被称为顶针，但它却是在中指上使用的。

youdao

The original meaning is that punishment for someone can serve as a warning to others. But people now use the idiom to indicate that premature actions which put the enemy on guard.

原先的意思是指对某人的惩罚成了对其他人的警告，但现在人们用它来比喻不成熟的行为会使敌人产生戒备。

youdao

Put none but Americans on guard tonight.

今夜只让美国人担任警戒。

youdao

By the same token, they put us on our guard, turn us into members of the jury.

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The school says the decision to put the guard on campus was supported by a committee comprised of parents, teachers and community leaders.

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Recalling my past frustrations, this immediately put me on my guard and I started the countdown to the inevitable blue-screen-of-death that would signal that some driver or another was wholly incompatible with some function or another of my laptop.

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