Geoff's example referred to the limited definition as including "physicians, lawyers, and clerics".

Question: I seem to recall that at university graduation ceremonies, doctoral robes are worn not only by the newly minted PhD's, but by others whose, such as M.D.'s and J.D.'s, whose degree required three or more post-graduate years. Could it be that limited meaning of "professional" is (or at one time was) tied to that three-year marker, referring to those who held a 3+ year post-grad degree in their field?