not to ofend...

[monkey16920]

where did the phrase "thumb up one's ass" come from?

[Quizmonster]

I suspect the phrase started its life as military slang. Any soldier who proved to be almost totally ineffective was accused of having "his thumb in his bum and mind in neutral." I suppose the idea behind it was that the thumb was a sort of gear-lever, only his one was always in the wrong place, so he never achieved any forward motion! The version you offer is probably the American one, using the alternative word for 'bum'.

[crete]

if anyone should know it would be you monkey

[shammydodger]

This is attributed (by me) to Oscar Wilde:

"There is only one thing in the world worse than being talked about, and that is having one's thumb up one's ass."