According to the OED it’s a translation of the name (awahkan) the Cree gave to those people which means slave or captive. It gets its disyllabic pronuciation from being a local jargon variant with the French suffix ‘ais’. OED also defines them as a group of Athapascan-speaking people from the boreal region of northwestern Canada and it is the word for their language as well.
Also according to OED the name for the group of people also gave the name to the Slave River and lake - the region where they lived. First cite in OED 1789