煞有介事 with great seriousness
Explanation
指装模作样，好像真有其事的样子。多指大模大样，好像很了不起的样子。
To act in a serious and formal manner as if something is truly important or significant, often implying a display of self-importance.
Origin Story
小明为了在同学面前炫耀，煞有介事地拿出一个旧盒子，声称这是他祖上传下来的宝物，里面装着稀世珍宝。他小心翼翼地打开盒子，里面却只是一些干枯的树叶。同学们都忍不住笑了，小明这才意识到自己过于夸张了。这个故事告诉我们，做事应该实事求是，不要为了虚荣心而夸大其词，煞有介事地欺骗他人。
To show off to his classmates, Xiao Ming solemnly took out an old box, claiming it was a treasure passed down from his ancestors containing rare treasures. He carefully opened the box, only to reveal some dried leaves. His classmates couldn't help but laugh, and Xiao Ming realized he had been overly exaggerated. This story tells us that we should be true to ourselves and not exaggerate or deceive others for vanity.
Usage
作谓语、定语、状语；多用于贬义；指装模作样，好像真有其事的样子。
Used as a predicate, attributive, or adverbial; often used in a derogatory sense; refers to acting in a serious and formal manner as if something is truly important or significant, often implying a display of self-importance.
Examples
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他煞有介事地向大家讲解他的“伟大发明”，其实不过是一个简单的装置。
tā shàyǒujièshi de xiàng dàjiā jiǎngjiě tā de "wěidà fā míng", qíshí bùguò shì yīgè jiǎndān de zhuāngzhì
He explained his 'great invention' to everyone with great seriousness, but it was only a simple device.
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他煞有介事地批评我的文章，语气之严厉，让我不知所措。
tā shàyǒujièshi de pīpíng wǒ de wénzhāng, yǔqì zhī yánlì, ràng wǒ bùzhī suǒcuò
He criticized my article with seriousness and severity, leaving me at a loss as to what to do