Man, I kept digging him on this one. Right out there in front of everybody he said it: everybody knew what he meant, but instead of "opposable thumbs" he said "prehensile thumbs." We all let him get a couple sentences further in before everyone started looking at me all expectant, so I had to start going to town on him on that one, on his prehensile thumbs. Do they curl all around like a monkey's tail?? And on through the evening, too, I'd keep bringing that one back in, like what the stand-up comedy pros like to call a "call-back." I kept calling that one back on speed-dial. With my prehensile thumbs.
Later we all found out, that's perfectly acceptable usage. But it was too late by then. We all still give him shit about it. In fact, it's even funnier now, because he actually has prehensile thumbs now. He can't even deny it!
I picture him with these furry thumbs, all long and curling around things to pick them up. What a freak.