Mid 17th century. Apparently from German Generalissimus, use as noun of post-classical Latin generalissimus, adjective, after Italian generalissimo. Compare Swedish generalissimus, French généralissime, both after Italian.
Definition of generalissimus in US English:
generalissimus
nounˌdʒɛn(ə)rəˈlɪsɪməs
Generalissimo.
Origin
Mid 17th century. Apparently from German Generalissimus, use as noun of post-classical Latin generalissimus, adjective, after Italian generalissimo. Compare Swedish generalissimus, French généralissime, both after Italian.