Definition from Wiktionary, the free dictionary
Uncertain, but likely from the fact that scrubbing is done while down on one's knees, just as in prayer.
holystone (plural holystones)
- (nautical) A block of soft sandstone used for scrubbing the wooden decks of a ship, usually with sand and seawater; sometimes called a bible.
- (Can we find and add a quotation of Totten to this entry?)
Smaller blocks for awkward places are prayer books.
- (transitive) To scrub the decks with a holystone.