admiralissimo
/ˌadm(ə)rəˈlɪsɪməʊ/ /ˌadm(ə)rlˈɪsɪməʊ/noun
(A title held by) a supreme commanding officer of a country's navy, or of combined naval forces.
Origin
Early 18th century; earliest use found in John Dart (d. 1730), antiquary. From admiral + -issimo.