Nightstick

According to Wikipedia, a truncheon is another name for a "Baton (law enforcement)", which may also be known as a cosh, a billystick, a billy club, a nightstick, a sap, a blackjack, or simply a stick.

Wikipedia goes on to explain the origin of the term 'nightstick': New York police used to use two types of baton – one in the day–time, and a longer one at night "so it could provide extra protection". The two types of baton were known, not unreasonably, as a day–stick and a night–stick.