Meaning

noun a soft sandstone used for scrubbing the decks of a ship

verb scrub with a holystone

originUncertain, but equivalent to holy + stone. As an amulet, probably from holey (“having a hole”). As a scouring stone, variously derived from holey, from the amulet, from its association with Sunday cleaning, from its users' adoption of a kneeling position similar to prayer, and (least likely) from their original provision by raiding graveyards for tombstones.

Etymology from Wiktionary, CC BY-SA 3.0