Possibly from Gaelic sliocht, 'offspring', pronounced 'slucht', in Irish usage an anglicised term referring to a dominant ruling kingroup (unconnected Irish kingroups could bear the same name, eg, there are multiple unrelated septs of Murphy, O'Connor, O'Kelly, etc, and a distinction should be made with regard to the Scottish 'clan system', where indeed 'sept' is used to denote a subdivision of a clan)