Definition of "shudderings"

Such an added, gliding strangeness began to invest the thin Fedallah now; such ceaseless shudderings shook him; that the men looked dubious at him […]

1851 November 14, Herman Melville, Moby-Dick; or, The Whale, 1st American edition, New York, N.Y.: Harper & Brothers; London: Richard Bentley