To be nonplussed by something is to be left confused, unsure, or baffled by it. The performance was so bizarre and meandering that it left me nonplussed.
If you're good at French or Latin, you'll recognize that "non plus" means "no more." When you're nonplussed by something baffling and bewildering, there's no more you can say or do about it. My goal in getting poor grades, running with a bad crowd, and refusing to eat was to nonplus my parents. I hoped they'd give up and finally agree to send me to boarding school. Sometimes people misuse nonplussed to mean "unimpressed," but that's not correct: to nonplus is to puzzle, confuse, and dumbfound.
