Amuck is the same word as amok. The spelling you will meet most now is amok; amuck is the older look, still used, especially in the phrase run amuck. It means to go wild and out of control — often in a frightening, even violent way. A person can run amuck; people also stretch it: “the rumour ran amuck.” It is not a mild “play around,” and it is not a medical word. Though actually, many teachers now ask students to learn amok as the main spelling and treat amuck as a twin they may see in older books.