Combination of having both hands horizontally out at chest level, palms up, fingers spread, elbows close to body and shrug - to mean I don`t know.
Has anybody else ever seen swiping the hand at the neck quickly and repeatedly in a horizontal motion to mean - urgent cut it short or shut up?Have you ever noticed that in English there is also a specific series of notes which mean "I don't know" when hummed? You might use it If, for example, your mouth is full and you don't want to get French fries all over the person who is asking you something. Of course the absolute pitch varies but you might write it Low-High-Middle, the same notes as saying "I don't know".
As for the cut it short or shut up thing - yes, it's a good one. Best done toward your buddy, as they talk to someone else (who can't see you), and your friend is about to put their foot in their mouth, that sort of thing. Vivid facial expressions help, too!
