In Stranger in a Strange Land, Robert Heinlein uses the word grok, a new word he coined, on page 22 without any definition or context. The word gets used frequently over the following 200 pages until it's given somewhat of a definition.

To grok something is to understand it intuitively and deeply, so much so that it becomes second nature. But giving a formal definition of grok misses the point.

There's something that draws computer scientists to the word (and I think it goes beyond the fact that most computer scientists are probably slightly more likely to enjoy science fiction than the average person.) Sure, you can write code in a specific language, but do you grok it? For programmers who use Python, there's a word for code written by coders who grok the language - Pythonic.

I don't know how long it takes to grok something. It depends. But I know you can't really grok it until you do it.

Below is an excerpt from Stranger in a Strange Land where one of the main characters finally describes grok:

“Now take this one word: ‘grok.’ Its literal meaning, one which I suspect goes back to the origin of the Martian race as thinking, speaking creatures—and which throws light on their whole ‘map’—is quite easy. ‘Grok’ means ‘to drink.’”
“Huh?” said Jubal. “But Mike never says ‘grok’ when he’s just talking about drinking. He—”
“Just a moment.” Mahmoud spoke to Mike in Martian.
Mike looked faintly surprised and said, “‘Grok’ is drink,” and dropped the matter.

“But Mike would also have agreed,” Mahmoud went on, “if I had named a hundred other English words, words which represent what we think of as different concepts, even pairs of antithetical concepts. And ‘grok’ means all of these, depending on how you use it. It means ‘fear,’ it means ‘love,’ it means ‘hate’—proper hate, for by the Martian ‘map’ you cannot possibly hate anything unless you grok it completely, understand it so thoroughly that you merge with it and it merges with you—then and only then can you hate it. By hating yourself. But this also implies, by necessity, that you love it, too, and cherish it and would not have it otherwise. Then you can hate—and (I think) that Martian hate is an emotion so black that the nearest human equivalent could only be called a mild distaste.

Mahmoud screwed up his face. “It means ‘identically equal’ in the mathematical sense. The human cliché, ‘This hurts me worse than it does you’ has a Martian flavor to it, if only a trace. The Martians seem to know instinctively what we learned painfully from modern physics, that the observer interacts with the observed simply through the process of observation. ‘Grok’ means to understand so thoroughly that the observer becomes a part of the process being observed—to merge, to blend, to intermarry, to lose personal identity in group experience. It means almost everything that we mean by religion, philosophy, and science—and it means as little to us as color means to a blind man.” Mahmoud paused. “Jubal, if I chopped you up and made a stew of you, you and the stew, whatever else was in it, would grok—and when I ate you, we would grok together and nothing would be lost and it would not matter which one of us did the chopping up and eating.”

And as a fun fact, grok might be the first Martian word ever coined.