from The American Heritage® Dictionary of the English Language, 4th Edition
- n. Nautical A blue flag with a white square in the center, flown to signal that a ship is ready to sail.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A blue signal flag with a white rectangle in the centre, signifying "P". When flown alone, indicates that a ship is ready to sail, requiring all crew members and passengers to return on board.
From the fact that it represents the letter P in the International Code of Symbols.(American Heritage® Dictionary of the English Language, Fourth Edition)