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blue peter (n.)

a nautical term for a blue flag having a white square in the center, hoisted at the fore royal masthead as a signal to report on board as the vessel is about to go to sea, attested by c. 1800, from blue (adj.1), but the significance of peter is uncertain and disputed. Two common guesses are that it is an abbreviation of repeater or that it stands for French partir

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Definitions of blue peter from WordNet

blue peter (n.)
a blue flag with a white square in the center indicates that the vessel is ready to sail;
From wordnet.princeton.edu