振振有词 Eloquent and self-righteous
Explanation
形容理直气壮，自以为理由充分，说个没完。
To describe someone who argues confidently and at length, believing their reasons to be fully sufficient.
Origin Story
话说唐朝时期，有个叫李白的诗人，他特别喜欢喝酒，而且酒量惊人。有一天，他和几个朋友在酒楼喝酒，喝到兴头上，李白突然指着一个酒杯，振振有词地说："这酒杯，它能盛酒，它能传酒香，它能记录我们这千杯不醉的豪情壮志，它就是个宝！"朋友们都笑了起来，说他喝多了。李白却不以为然，继续振振有词地讲着酒杯的好处，从材质到造型，从历史到文化，滔滔不绝，最后还写了一首诗来赞美酒杯。朋友们被他这番振振有词的言论逗乐了，纷纷举杯，继续畅饮。虽然朋友们觉得李白有点夸张，但是谁也不能否认，李白确实很热爱生活，也很善于表达自己的情感。
During the Tang Dynasty, there was a poet named Li Bai who loved to drink and had an amazing capacity for alcohol. One day, while drinking with friends at a tavern, Li Bai suddenly pointed to a wine cup and said with great conviction, "This wine cup, it can hold wine, it can spread the aroma of wine, it can record our unwavering enthusiasm and lofty aspirations, it is a treasure!" His friends laughed, saying that he was drunk. Li Bai, however, was unfazed and continued to eloquently praise the virtues of the wine cup, discussing its material and design, its history and culture, endlessly. He even wrote a poem in its honor. Li Bai's friends were amused by his impassioned speech and raised their cups to continue the drinking spree. Although they found his praise somewhat exaggerated, they couldn't deny that Li Bai truly loved life and was very adept at expressing his emotions.
Usage
通常作谓语、定语、状语，形容理直气壮的样子，自以为理由充分，说个没完。
Usually used as a predicate, attributive, or adverbial modifier, describing someone who argues confidently and at length.
Examples
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他振振有词地为自己辩解，却无法改变事实。
tā zhèn zhèn yǒu cí de wèi zìjǐ biànjiě, què wúfǎ gǎibiàn shìshí
He argued his case eloquently, but it did not change the facts.
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面对证据，他仍然振振有词，令人气愤。
miànduì zhèngjù, tā réngrán zhèn zhèn yǒu cí, lìng rén qìfèn
In the face of evidence, he still argued his case vehemently, infuriating everyone present.