Definition of swan-upping noun from the Oxford Advanced Learner's Dictionary

 

swan-upping

 noun
noun
BrE
 
; NAmE
 
[uncountable]
 
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a ceremony that takes place every summer on the River Thames, in which groups of people in boats mark swans (= large white water birds with long necks) to show who owns them. Most swans in Britain belong to the queen, but in the 15th century a few groups of swans on the Thames were given to two of the City of London's livery companies. The young birds descended from these groups are marked each year to distinguish them from the royal swans.