Salutation; refers to the artificial horizon indicator on an aircraft instrument panel. Blue represents sky and black represents the ground. If the black side is up, this will not end well. Similar to truckers' salutation of "Keep it between the paint" and satirical theatrical salutation of "Break a leg."
Said to a spouce: "See ya later, honey; keep the blue side up!"
To a friend: "Remember to keep the blue side up!"
When you have to pee in a movie but don’t want to miss anything, and must do the following mental equation: will the amount I have to pee cause enough discomfort to diminish my attention capacity for the remainder of the movie to an extent which is greater than the amount of the movie I would miss by making a mad dash to the restroom.
I saw Satantango last night, wonderful film but by the 5th hour I had to start doing pee calculus.
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).