Source : Webster's Revised Unabridged Dictionary (1913)

Holystone \Ho"ly*stone`\, n. (Naut.)
   A stone used by seamen for scrubbing the decks of ships.
   --Totten.

Holystone \Ho"ly*stone`\, v. t. (Naut.)
   To scrub with a holystone, as the deck of a vessel.

Source : WordNet®

holystone
     n : a soft sandstone used for scrubbing the decks of a ship
     v : scrub with a holystone; "holystone the ship's deck"