sinn fein·ism \|shinˈfāˌnizəm\noun (-s) Usage: usually capitalized S&F Etymology:Sinn Fein, national Irish society founded about 1905 (from Irish Gaelic sinn fēin we ourselves) + English -ism 1.: the doctrines, policies, or practices of Sinn Fein 2.: a movement based on the doctrines of Sinn Fein