(third-person singular simple present clams up, present participle clamming up, simple past and past participle clammed up)
By analogy with a clam closing itself.
"clam-up." YourDictionary, n.d. Web. 23 June 2018. <http://www.yourdictionary.com/clam-up>.
clam-up. (n.d.). Retrieved June 23rd, 2018, from http://www.yourdictionary.com/clam-up
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