One of the mysteries of the English language finally explained.
Of a sail: positioned so as to be filled by the wind; taut and full. Of a sailing ship, etc.: sailing with full sails. Also as interjection: an order to sail in this manner.
A state or position of sailing in which the sails of a craft are taut and full. Frequently in "to give (a sail or vessel) a rap full" and variants.
Early 18th century. Apparently from a first element of unknown origin + full; compare -ful. With use as noun perhaps compare e.g. handful.
rap full/ˌrap ˈfʊl/
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