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Meaning of mail flag

mail' flag"

Pronunciation: [key]
  1. a flag of the International Code of Signals symbolizing the letter Y, flown alone by a ship to indicate that it is carrying mail: a square flag with red and yellow diagonal stripes.
  2. any of various other flags indicating that a ship is carrying mail.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.