From French Esclave, a calque of a Cree name, as the Cree called these people (and several other traditional enemies) slaves. In English, the final e came to be pronounced as if Slave were a native name; this pronunciation was first written Slavé and later Slavey. The names of the Slave River, Lesser Slave River, Great Slave Lake and Lesser Slave Lake do not necessarily derive from the people now known as Slavey in English.

Etymology: Wiktionary (CC-BY-SA)