It is a locution associated with exposing an idea omitting some concepts, rather out of distraction or interest than ignorance. It comes from letters, newspaper articles, historical novels, which in other times were the main way of transmitting information or even education, and which were written with a pen, wetting the tip in the inkwell. It's actually an irony, like "something was left unwritten." See other versions already published such as keeping someone something in the inkwell, leaving someone something in the inkwell.
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