Boise's name comes from the French word boise, which means "wooded". Some people assume that the word means tree, but the French word for "tree" is arbre. The word bois means "wood". It's said that French Canadian fur trappers came over the mountains looked down upon the Boise River Valley in the early 1800s and exclaimed "Les bois!" (The wood!), so that is how Boise gained its nickname 'The City of Trees'
It is believed that the area was called Boise long before the establishment of Fort Boise. No one knows for sure the exact details of the how the name was applied.
Some say that Captain BLE Bonneville was the source of the name. After a long trek across the dry rough high desert, the exploration party reached a high point that overlooked the Boise River valley. That place is now called Bonneville Point, and it is part of the Oregon Trail just east of Boise. The story goes, one of the French speaking guide's, was so excited at the site of the river, he started yelling "Les bois! Le bois!" giving the area its name.
The Name "Boise" may actually derive from the Mountain Men, after naming the river that flows through it. Around 1820 French Canadian fur trappers set trap lines in the area where Boise now is. The area is high desert so the tree lined Boise river made it a prominent landmark. The French Canadian's called it "La rivere boisee", when translated means "The Wooded River".
Fort Boise was located about 40 miles down river near its meeting with the Snake River on the Oregon border. The fort was built by the Hudson Bay Company in the 1830's. It was abandoned in the 1850's. After massacres along the Oregon Trail the US Army re-established a fort in 1863, during the US Civil War. The location was selected because it was at the intersection of the Oregon Trail and the roads going the Boise Basin, Idaho City, and the Owyhee Mining areas. Both areas were booming, Idaho City was the largest city in Idaho at that time. Fort Boise grew rapidly as a staging area to Idaho City. Boise became a city in 1864. Lewiston was the first capital of Idaho but Boise replaced it as the capital in 1865.