How to explain what the eff in ineffable means without going to Latin

The word ineffable is, at first glance, very simple.  Prefix-Root-Suffix.

"In[blank]able" means, "unable to be [blanked]".  Simple enough.

In the most recent Edith and Ben I had Ben use this knowledge to back-form the word "evited" from "inevitable".

The trouble with doing things like this is that no one knows what the fuck "evit" means unless they're starting with "inevitable" in mind.  Similarly "kempt" is a word (turns/turned 85 sometime this year) that you only understand if you know "unkempt".

But what about when you don't know the original word?  How do you explain "ineffable" to someone who doesn't already know what it means?

Well the basics are already there.  "ineffable" means "unable to be effed".  All one needs to do is explain what "eff" means and you're done.  For those of us with our minds not in the gutter, though, there's a problem.

"Eff" is not now, nor has it ever been, an English word.  "Effable" is, even if my spell checker doesn't recognize it, but that just means, "able to be effed."

Now is the time when we could go to the Latin, point out that the "eff" here comes from "effor" which is "ex"+"for" and thus originally meant "to speak out".  BUT, and yes this deserves an all caps "but" at the beginning of a sentence thus breaking all rules of grammar and tact, we would then need to explain that given a couple thousand years the meanings of words can change, a lot.

We'd have to talk about how it originally meant that but the meaning became more metaphorical and in fact deeper and more encompassing than anything involving mere speech.

That's a lot of work to deal with a three letter root.  Isn't there an English word that means the same thing "eff" means now?

Well, yes.  There is.  But there's a catch: it's not an Earth word.  The word is "grok".

Something is ineffable if it is not able to be grokked.
Something is effable if is is able to be grokked.

Thank the Martians, they've given us a simple way to explain what ineffable means:

-What does it mean when something is ineffable?
-It means that you can't grok it.

The entire idea of ineffability is that some things can't be grokked by our human brains.  Perhaps not by any brains.  You can study it, you can go for a deeper understanding, you can approach the problem from every angle, but you will never, ever grok it.

And that's what the "eff" in ineffable means.  It means "grok".  In real etymology "eff" started as speaking out.  In fictional etymology "grok" started as drinking, but in both they arrived at the same place.  Understanding in fullness.

If something is truly ineffable you might be able to get parts of it, you might have some understanding of details around the edges, but you will never truly get it.  Partial understanding, maybe.  Grokking, no.