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Meaning of sand jack

sand' jack"

Pronunciation: [key]
— Shipbuilding. Shipbuilding.
  1. any of a number of containers of sand driven beneath a hull about to be launched as a temporary support and then drained of sand so as to let the hull down onto the launching cradle.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.