The term "buck up" is thought to have originated in the 14th century, when it was used in a nautical context to refer to the action of raising or securing a ship's sails. It is believed that the term was derived from the Old Norse word "bukkr," which means "male goat," and was used to describe the action of a goat standing up on its hind legs. Over time, the term came to be used more generally to refer to the action of standing up or bracing oneself, both physically and mentally.