Guys, at this point I’m 99% positive that this is just Cas’s sense of humor coming out.
I think Cas fully understands what Dean means when he asks “What’s the word, Cas?”
Cas’s sense of humor = amusement at absurdities. The tower of Babel was a pile of dung. I comforted the pig before the slaughter. The coyote is man and the roadrunner is God, trapped in an endless cycle. What the hell? Guess again. You breed with the mouth of a goat.
Cas understands language and Dean’s slang just fine. This is his idea of a joke.
And his delivery is always so deadpan that it’s hard to tell when he’s making a joke. Cas’s sense of humor is really rather subtle, and I find it freaking hilarious that people think he doesn’t understand sarcasm or jokes. Sure he does. He uses sarcasm several times over the course of the series. Granted, he doesn’t always use it correctly, especially when he’s having trouble actually communicating what he’s trying to say - I think he gets flustered - but he gets it.
His sense of humor is just a trifle offbeat. I think that’s probably just what happens when you’re thousands of years old. Obscure things start seeming really fucking hilarious.
And Dean’s reaction? I think he kinda looks like he’s not sure if Cas is joking or not, but just to be on the safe side, he goes ahead and rephrases his question.
The real tragedy of this show is that nobody gets Cas’s jokes.