Expression of the day - Nothing Doing

Read it in a cartoon today; never heard it before in my life.
Evidently "Nothing Doing" is an expression of refusal meaning no hope of sucess.

Here's another expression I have always been wondering about.
"the situation was all but tense" - when does it mean that the situation really is tense, and when does it mean that the situation is everything BUT tense?

I am sure I have some better example written up somewhere on a chewing gum paper or something, because it has been riding me for years, but I cant come up with a better example right now - will edit back when I find the appropriate post-it.