People who propel a flat-bottomed boat with a pole, or in British slang, people who bet on horses or sports, or ordinary customers.
From 'punt,' a flat-bottomed boat used on rivers, from Dutch 'ponton.' The term expanded to mean people who bet (punters on horse races) and then casual customers in general.
In Venetian gondola-style punting, the pole must touch the river bottom or you're in trouble—professional punters on the Thames can propel boats at surprising speeds despite looking leisurely!
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