The word "generalissimo" comes from the Italian generale, general, plus the suffix -issimo, meaning "utmost, to the highest grade".The rank was historically given to a military officer leading an entire army or the entire armed forces, only subordinated to the Sovereign. "Generalissimo" is sometimes used in modern English language to refer to a military officer who has obtained political power by a military coup, or in some cases one who has suspended pre-existing constitutional mechanisms in order to retain power by means of a military hierarchy.
From 1834 to 1910, the Kings of Portugal were considered "Generalissimo", in their constitutional role of Supreme Commanders of the Portuguese Army.
There were four holders of the Russian rank or title "generalissimus" prior to the 20th century. Menshikov both commanded military forces and ruled absolutely; Aleksei Shein and Aleksandr Suvorov, were principally field commanders rather than political figures. Anthony Ulrich II, Duke of Brunswick-Lüneburg (1714 - 1776), was appointed generalissimus by his wife Anna Leopoldovna but neither commanded nor ruled.
On Oct. 11, 1976, by Act of Congress, Washington was posthumously appointed to the grade of "General of the Armies of the United States". The appointment was backdated to July 4, 1776 and was a designation that, by law, made George Washington the most senior United States military officer, past and present.