Rashi: It is the name of a place. Radak - it was called so, for the pit was in a place of thorns. ¹

Rashi and Radak, from Sanhedrin 49a: A pit and a thorn caused Avner be killed - he did not support David [who proved that he does not want to kill Sha'ul] through the flask of water that he took from near Sha'ul's head , and from the corner of Sha'ul's garment that he cut. Avner gave poor rejections. Perhaps David did not have opportunity to kill you; a servant took your flask to bring water from a pit, and forgot it there. ² Your robe was cut on a thorn, and David found the piece cut off!

Kli Yakar, Malbim: Yo'av sent immediately, so he will think that David sent them. The verse teaches that he did not know.