According to Jessica Snyder Sachs in the forensic book Corpse,

History records the first known application of medical knowledge to death investigation in 44 B.C. Summoned to examine the body of Julius Caesar, the Roman physician Antistius announced that he knew which of the would-be emperor's twenty-three stab wounds had proved fatal. By clocking death to a particular blow, Antistius thwarted the plot by which the Roman senators had hoped to avoid any one of them standing trial for murder. In the end, history tells us, they all paid with their lives. But Antistius's historic death determination, however dubious it may have been, marked the beginning of the pathologist's role as expert witness to murder. In fact, it gave us the term forensic, Latin for "before the forum," which is where Antitius made his fateful declaration.

I haven't checked other sources to determine the validity of this story, but forensic is definitely from forensis, "of a forum, place of assembly," from forum.