A test, usually quite random, to determine how lucky you will be that day. Your luck is determined by how unlikely the thing you predict to occur in the next time period, event, etc. is. The term comes from the first, and most common test, which is dropping a piece of buttered toast on a tabletop. If it lands butter side down, bad luck day. Butter side up is a good day. On it's side means that you should go buy a lottery ticket asap.
It's closely related to the spoon test which is when one puts a spoon under their pillow and then prays for a certain kind of weather the next day. The difference is that no "luck" is achieved by the spoon test, just a certain outcome.
Ancient, archaic forms of this test include plucking daisy petals and saying "he loves me, he loves me not", and breaking wishbones

the thing that you don’t have when you’re phones percentage is over 20