Accounts differ about the origin of the name. One account credits Capt. B.L.E. Bonneville of the U.S. Army as its source. After trekking for weeks through dry and rough terrain, his exploration party reached an overlook with a view of the Boise River Valley. The place where they stood is called Bonneville Point, located on the Oregon Trail east of the city. According to the story, a French-speaking guide, overwhelmed by the sight of the verdant river, yelled "Les bois! Les bois!" ("The trees! The trees!")—and the name stuck.
The name may instead derive from earlier mountain men, who named the river that flows through it. In the 1820s, French Canadian fur trappers set trap lines in the vicinity. Set in a high-desert area, the tree-lined valley of the Boise River became a distinct landmark. They called this "La rivière boisée", which means "the wooded river."
Natives, and those who have lived in the area for a long time, use the pronunciation /ˈbɔɪsiː/ (BOY-see). This is the pronunciation given on the city's website. The pronunciation is sometimes used as a shibboleth by native Boiseans and other longtime residents, as outsiders tend to pronounce the city's name as /ˈbɔɪziː/ (BOY-zee). Note that there is considerable controversy around this.
Boise is often called the "City of Trees." Bobo is another popular nickname for Boise, Idaho.