there are many definitions for flipping the mustard; the most popular being to laugh so hard until you pee. Also, to flip the mustard can mean to grow angry and throw a fit, or conversely to have a startling revelation/epiphany which makes total sense of a previously bewildering topic. In addition, it can mean to literally flip the mustard container on your dinner table.
Did you see that? I think I just saw Jack flip the mustard!
I'm surprised we haven't seen John flip the mustard yet...
Man, David's being quite the @^$*(&# lately... he flips the mustard quite often.