A thumb screw is originally (and, from all its meanings, my personal favourite) a torture device. It was screwed over someone's thumb 'till it slowely broke (duh!).
I was discussing the similarity between a nut-cracker (another good name for a torture device) and a thumb-screw with a friend, when she gave me a strange look. "Don't you know what a thumb-screw is?", I asked. My friend, who took the term too literally, would not stay behind and answered something like "duh! Don't you know what sit on my face is?"