Longbow +1

A Longbow +1 is an enchanted weapon, which has a -2 THAC0 and a speed factor two points lower when compared to a normal longbow.

Baldur's Gate II: Shadows of Amn, Throne of Bhaal and the Enhanced Edition

 * Adventurer Mart - Ribald Barterman
 * Karthis al-Hezzar - North Forest
 * Tankard Tree - Saradush

In-game description
"The longbow is similar to the shortbow, except that the staff is about as tall as the archer is—usually six to six-and-a-half feet. It has better range and accuracy than the shortbow, but a slower Speed Factor. The smooth staff of this bow is warm to the touch, just a hint of the energy that surges through the wood."