Lincoln var drepinn þar..

An early use for the word “cool” in such a context was Abraham Lincoln.

Under all these circumstances, do you really feel yourselves justified to break up this Government unless such a court decision as yours is, shall be at once submitted to as a conclusive and final rule of political action? But you will not abide the election of a Republican president! In that supposed event, you say, you will destroy the Union; and then, you say, the great crime of having destroyed it will be upon us! That is cool. A highwayman holds a pistol to my ear, and mutters through his teeth, “Stand and deliver, or I shall kill you, and then you will be a murderer!”