Get hold of yourself, calm down. This imperative had several earlier meanings; one, dating from the 1940s in the military, was to put effort into what one was doing. Another, in college slang a few decades later, was to pay attention. The current meaning, however, is the one that has survived. In a Boston Globe editorial (Oct. 31, 2004) describing former New York City mayor Ed Koch’s complaint that the Democratic National Convention in Boston “had no excitement,” the writer said, “Get a grip, Ed. It was only our first one. Beginner’s luck.” The usage is the converse of to lose one’s grip, to lose composure, first recorded in 1875 and cited by the OED. See also get a handle on something.