To nonplus is to baffle or confuse someone to the point that they have nothing to say. Something weird and mysterious can nonplus you, like a play that is performed entirely by chickens.
If you know a little French or Latin, you'll recognize that "non plus" means "no more." When something bewildering nonpluses you, there's no more you can say or do about it. A goal of getting poor grades, running with a bad crowd, and refusing to eat would leave your parents nonplussed. Sometimes people misuse nonplus to mean "unimpressed," but that's not correct: to nonplus is to puzzle, confuse, and dumbfound.