Definition of "unavailing"

The Begums' ministers, on the contrary, to extort from them the disclosure of the place which concealed the treasures, were, […] after being fettered and imprisoned, led out on to a scaffold, and this array of terrours proving unavailing, the meek tempered Middleton, as a dernier resort, menaced them with a confinement in the fortress of Chunargar. Thus, my lords, was a British garrison made the climax of cruelties!

1788 June, Richard Brinsley Sheridan, “Mr. Sheridan’s Speech, on Summing Up the Evidence on the Second, or Begum Charge against Warren Hastings, Esq., Delivered before the High Court of Parliament, June 1788”, in Select Speeches, Forensick and Parliamentary, with Prefatory Remarks by N[athaniel] Chapman, M.D., volume I, [Philadelphia, Pa.]: Published by Hopkins and Earle, no. 170, Market Street, published 1808, page 474