Boundary County, Idaho: Boundary is named for the fact that it borders Washington on the west, Montana on the east, and Canada (British Columbia) on the north.
Butte County, Idaho: Butte is named for the buttes scattered throughout the county.
Camas County, Idaho: Camas is named for the local camassia plant.
Caribou County, Idaho: Caribou is named for the Caribou Mountains.
Cassia County, Idaho: Cassia is named for Cassia Creek, which flows through the center of the county into the Raft River.
Clark County, Idaho: Clark is named for Sam K. Clark, an Idaho state legislator from the county.
Clearwater County, Idaho: Clearwater is named for the Clearwater River, which flows into the Snake River at Lewiston, Idaho.
Custer County, Idaho: Custer is named for the General Custer mine, in turn named after General George Armstrong Custer.
Elmore County, Idaho: Elmore is named for the Ida Elmore mines.
Franklin County, Idaho: Franklin is named for Franklin, Idaho, the first town in the state, in turn named for Franklin Richards, an apostle of the Church of Jesus Christ of Latter-Day Saints.
Fremont County, Idaho: Fremont is named for John Charles Frémont, the famous explorer and U.S. Senator.
Gooding County, Idaho: Gooding is named for Frank Robert Gooding, the seventh governor of Idaho.
Idaho County, Idaho: Idaho is named for ee-da-how, an invented word supposedly meaning the sun comes from the mountains in the Shoshone language. Ee-da-how is also the state name's origin.
Oneida County, Idaho: Oneida is named for the Oneida Lake area of New York from which many early settlers hailed.
Owyhee County, Idaho: Owyhee is named for the Kingdom of Hawaii. Hawaiian fur trappers had explored (and had been mysteriously killed in) the area around 1820.
Payette County, Idaho: Payette is named for the Payette River, which, in turn, is named for explorer Francois Payette.