Definition of "inscrutable"

The pale, inscrutable determinateness, and flinchless intrepidity of Pierre, now began to domineer upon them; for any social unusualness or greatness is sometimes most impressive in the retrospect.

1852 July, Herman Melville, “Book XXIV. Lucy at the Apostles’.”, in Pierre: Or, The Ambiguities, New York, N.Y.: Harper & Brothers, […], section IV, page 445

It was unconscious, this smile was, though just after he had said something it got intensified for an instant. It came at the end of his speeches like a seal applied on the words to make the meaning of the commonest phrase appear absolutely inscrutable.

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 211, column 1

She called my vanity license plate inscrutable! 'ICU81MI'? Hilarious!

2007 October 11, Robert Carlock, “Jack Gets in the Game”, in 30 Rock, season 2, episode 2, spoken by Tracy Jordan (Tracy Morgan)