Definition of "alienist"

‘Are you an alienist?’ I interrupted. ‘Every doctor should be - a little,’ answered that original, imperturbably.

1899 February, Joseph Conrad, “The Heart of Darkness”, in Blackwood’s Edinburgh Magazine, volume CLXV, number M, New York, N.Y.: The Leonard Scott Publishing Company, […], part I, page 201

A trial for poisoning means an exhaustive study not only of analytic chemistry, but of practical medicine on the part of all the lawyers in the case, while a plea of insanity requires that, for the time being, the district attorney shall become an alienist, familiar with every aspect of paranoia, dementia praecox, and all other forms of mania.

1912, Arthur Cheney Train, “Preparing a Criminal Case for Trial”, in Courts and Criminals‎

“Speaking as a medical man,” said I, “it appears to be a case for an alienist. The old gentleman's cerebral processes were disturbed by the love affair.”

1923 March, Arthur Conan Doyle, “The Adventure of the Creeping Man”, in The Case-Book of Sherlock Holmes, London: John Murray, […], published June 1927 (May 1952 printing), page 212

There was probably not an alienist in the land who, having listened so far, would not have sprung at George and held him down with one hand while with the other he signed the necessary certificate of lunacy. But Molly Waddington saw deeper into the matter.

1927 April 28, P, published 1950