I always took it as simply as “Why do you keep saying things aren’t possible/probable when they keep happening one after the other? Maybe you’re just not as smart a planner as you thought?”
It was just a dig that Vezzini kept covering for his lousy planning and overblown sense of intelligence by claiming that none of the problems could have been imagined.
That’s how I always understood it also. And quite frankly any other interpretation is just, just, just …(wait for it)…
inconceivable! 
He only squirms to bluff Vizzini into thinking he’s winning.
You’re trying to trick me into giving something away…
Won’t work.
It has worked! You’ve given everything away!
Look! Over there!
“I know exactly which [del]glass[/del] cup the poison is in. It’s in - what in the world is that?!?!?!”
Also as a Spaniard Inigo obviously still has a little trouble with the English. That’s why its funny he is the one telling Vizzini he is using the word wrong.
nope – it’s “…what in the world can that be?”
Nice try, though. 
Jophiel:
Agreed.
hrmph
well, I was a lot closer than simster, anyway 
It can’t be a rodent of unusual size… I don’t believe they exist.
I’m not one to be trifled with, that’s all you ever need know.
ANYWAY -
So, You’ve decided then?
Not remotely.
Am still not getting it. God dammit I want to, but am not.
On the other hand, the picture of Mandy Patinkin has him looking the best he has in a few years, IMO - the couple episodes of Criminal Minds I’ve seen, he’s looked about to collapse in on himself.
Elwes and Crystal both seem to have put on some weight, and seem to have been caught in an awkward pose, with their heads pulled back, which really accentuates the fat in their faces.
Let me explain…
…no, there is no time. Let me summarize.
(Now I have to see if I have time to watch it before I head off to work. checks watch)
**Let me 'splain…
…no, there is too much. Let me sum up.**
People people. If you haven’t seen it at least 100 times, . . . .
My absolute FAVORITE line is
“Your strength, his steel, and my brains against 60 men, and you think a little head jiggle is going to make me happy? Hmmmm???”
Now, if we only had a holocost cloak…
I never saw as a dig- just that Fezzik was trying to be helpful- maybe the word didn’t mean what Vezzini thought it did because the opposite of the inferred meaning (to Fezzik) was coming to pass. He just wanted to point that discrepency out to Vezzini, in case Vezzeni was wrong.
I always saw Fezzik as earnest.