What Alexander Hamilton meant was that out of the three branches of the government (Executive, Legislature and the Judiciary), the judiciary (Supreme Court) has almost no power to exercise on it's own, thereby making it "the least dangerous branch".
Explanation:
When Hamilton wrote that the supreme court was the least dangerous branch of government, he meant to say that the court has no authority or power to exercise very the other executive or legislature, nor can it act without these two. In short, is is nothing without these two. While these two can do things without it, the same isn't for the judiciary. The judiciary branch is the weakest branch of the government as it has no influence over either "the sword or the purse", without any force or will to exercise it's powers. It's basically an unnecessary need. Yes, what he said is true due to the fact that no laws can be passed without the consent of the other two branches, while things can be done without the knowledge of the judiciary. Courts need the permission of the Executive, the President or the Head of the Country to make any decision.
