Why was Roosevelt's policy called the "Big Stick" policy?

Big Stick policy, in American history, policy popularized and named by Theodore Roosevelt that asserted U.S. domination when such dominance was considered the moral imperative. Roosevelt’s first noted public use of the phrase occurred when he advocated before the U.S. Congress increasing naval preparation to support the nation’s diplomatic objectives.

Explanation:

Informally, the Truman Doctrine was generalized to become the foundation of American Cold War policies in Europe and around the world. It changed American foreign policy against the Soviet Union from détente (tension reduction) to containment (as proposed by diplomat George Kennan).