Hot answers tagged etymology
Actually this association comes from Japan, as written in 百度百科：招财猫. The article says there are stories of cats repaying their masters in Japan since Edo period and this is the origin of different kinds of 招财猫s. There is also a movie Neko no ongaeshi about this kind of stories.
I don't remember what the story was, but this phrase is about 张仪。You should probably read 史记 or 战国策。张仪 is the student of 鬼谷子。And the story has something to do with a failed 游说。I am sorry I can't provide more details. But even if I do, I am not sure I can tell the story in English, I am not good at it.
The original allusion was back in the Period od Warring States as deutschZuid has said. "三寸之舌，强于百万之师". Now as to the question of why use 三寸. This is apparently the usage of parallelism which is one of the most common rhetoric in ancient China: [三(number)寸(unit)之舌(noun)]，强于[百(number)万(unit)之师(noun)]. Although 百万 is actually a number, it can be interpreted as ...
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