河漢

The figurative sense “empty talk” is from Zhuangzi:

肩吾問於連叔曰：「吾聞言於接輿，大而無當，往而不反。吾驚怖其言，猶河漢而無極也。」 [Classical Chinese, trad.]

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肩吾问于连叔曰：「吾闻言于接舆，大而无当，往而不反。吾惊怖其言，犹河汉而无极也。」 [Classical Chinese, simp.]

From: Zhuangzi, circa 3^rd – 2^nd centuries BCE, translated based on James Legge's version

Jiānwú wèn yú Liánshū yuē: “Wú wén yán yú Jiēyú, dà ér wú dàng, wǎng ér bù fǎn. Wú jīngbù qí yán, yóu héhàn ér wújí yě.”

Jian Wu asked Lian Shu, saying, 'I heard Jie Yu talking words which were great, but had nothing corresponding to them (in reality); once gone, they could not be brought back. I was frightened by them; they were like the Milky Way which cannot be traced to its beginning or end.'

河漢

1. (literary) the Yellow River and the Han River
2. (literary or Taiwanese Hakka) Milky Way

   • 河漢清且淺，相去復幾許。 [Classical Chinese, trad.]
     河汉清且浅，相去复几许。 [Classical Chinese, simp.]

     From: 古詩十九首之十, in Wen Xuan

     Héhàn qīng qiě qiǎn, xiàngqù fù jǐxǔ.

     (please add an English translation of this quotation)

3. (literary, figuratively) something wide-ranging and profound
4. (literary, figuratively) empty talk

河漢

1. (literary, figuratively) to not believe; to ignore (someone's words)