Perhaps from the old practice of knocking under the table with the knuckles, when conquered.
"knock-under." YourDictionary, n.d. Web. 19 January 2019. <https://www.yourdictionary.com/knock-under>.
knock-under. (n.d.). Retrieved January 19th, 2019, from https://www.yourdictionary.com/knock-under
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