雷同

The Eastern Han Confucian scholar Zheng Xuan (鄭玄), in his annotations to the Book of Rites (《禮記》), derived the meaning as "to echo each other [同 (OC *doːŋ)] in the manner of thunder [雷 (OC *ruːl)]", writing that "when a thunderclap strikes, all things resonate (with it) simultaneously" (「雷之發聲，物無不同時應者。」). The original text so commented upon, referring to the proper etiquette when receiving a lecture from an elder, reads

長者不及，毋儳言。正爾容，聽必恭。毋勦說，毋雷同。 [Classical Chinese, trad.]

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长者不及，毋儳言。正尔容，听必恭。毋剿说，毋雷同。 [Classical Chinese, simp.]

From: The Book of Rites, c. 4^th – 2^nd century BCE, translated based on James Legge's version

Zhǎngzhě bùjí, wú chányán. Zhèng ěr róng, tīng bì gōng. Wú jiǎoshuō, wú léitóng. [Pinyin]

If there be any subject on which the elder has not touched, let him not introduce it irregularly. Let him keep his deportment correct, and listen respectfully. Let him not appropriate (to himself) the words (of others), nor (repeat them) as (the echo does the) thunder.

雷同

(set phrase)

1. to be identical (when that should not be the case); to mirror each other; to resemble

   如有雷同，純屬巧合。 [MSC, trad.]
   如有雷同，纯属巧合。 [MSC, simp.]

   Rú yǒu léitóng, chúnshǔ qiǎohé. [Pinyin]

   Any resemblance to actual individuals or events is purely coincidental.

2. to echo what others say; to parrot