As with Powhatan, little is known of Tamanend's life beyond his encounters with 17th century English settlers, specifically the 1682/3 Treaty of Shackamaxon allegedly forged between his Lenni-Lenape Native Americans and William Penn in modern-day Philadelphia.
This is a way of noting the date that historians use when it is not exactly certain in which year an event took place. For instance, if all available sources say that a treaty was signed "that winter," it is not clear whether the signing happened in late 1682 or in early 1683.
It can also happen that the event is only dateable to a year in a different calendar that doesn't change over on January 1st of our calendar. The ancient Athenians counted years by the tenure of a magistrate called the eponymous archon, who served for one year starting in July; that means an event that happened "in the archonship of X" could fall anywhere from the last half of one of our calendar years and the first half of the next. We would need more information to assign an event to one year or the other.
In such cases, historians will often abbreviate the date to 1682/3 (or, in my own field, 387/6 BC for example).