I looked it up, and that last post should have read "thumbs down," not "thumb's down," at least according to the Dictionary application on my Mac.
I'm reposting rather than editing because I thought you guys should know. That's weird, right? I would have thought it was "thumb's down," as in "thumb is down" or even "the down of the thumb."
But, what do I know? It's 3am and I've been on a plane for 6 hours. Me sleepy.