"exert, v." OED Online. Accessed 26 June 2019.
Can someone please expound the sentence underlined in green?
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Attaching ex "out" to serere "to bind" isn't really an obvious formation: what does it mean to "bind something out"?
So more likely, what happened is: in-serere "to bind in" became common, becoming the ancestor of modern English "insert" and taking on that meaning. And then when people needed an opposite for "insert", replacing in- with ex- was an obvious choice.
So ex-serere arose, meaning "to bring out", as in the opposite of "insert". Over time this evolved into the modern meaning of "exert".