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Related to nun buoy: can buoy
(Nautical Terms) nautical a buoy, conical at the top, marking the right side of a channel leading into a harbour: green in British waters but red in US waters. Compare can buoy
[C18: from obsolete nun a child's spinning top + buoy]
an unlighted buoy having a conical form above water.
[1695–1705; obsolete nun spinning top]