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.
any of various other flags indicating that a ship is carrying mail.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
How to use mail flag in a sentence
"This is a United States mail boat," and he pointed to the mail flag flying at the stern.The Boy With the U. S. Survey | Francis Rolt-Wheeler