Fast to act or react. The term comes from the gunslingers of the American West and was transferred to other kinds of quick reaction in the first half of the twentieth century. The literal meaning of quick on the trigger is a century older, appearing in a letter of 1808: “I trust that all your Aids will be quick on the trigger” (M. L. Weems, in E. E. F. Skeel, M. L. Weems: Works and Ways).