Short answers, pulled from the story.
Generalissimo is the absolute superlative of the Italian word generale, meaning "the highest-ranking of all generals." The suffix -issimo derives from the Latin -issimus, meaning "utmost, to the highest grade." Cognates appear in Spanish, Portuguese, French, and Latin.
Albrecht von Wallenstein became the first imperial generalissimo of the Holy Roman Empire in 1632. He was described as the general of the generals in that role.
Ferdinand Foch received the title généralissime on the 26th of March 1918, when Allied commanders needed unified authority on the Western Front. He held the rank of général de division and the dignity of Marshal of France, and later earned the ranks of British field marshal and Marshal of Poland.
Some sources assert that Stalin refused to accept the rank, but the grade was in fact established by the Presidium of the Supreme Soviet, which did not require his approval. Western diplomacy also used the rank of generalissimus to describe him.
The association developed largely because of Francisco Franco, who held the title generalísimo across all three branches of the Spanish armed forces from 1936 until 1975. The rank came to be linked with military officers who seized dictatorial power, and it appears in literature depicting fictional Latin American dictatorial regimes.
Yes. When Washington was chosen as commander-in-chief of the Continental Army in 1776, The Virginia Gazette called him the generalissimo of American forces. He was later promoted posthumously to General of the Armies of the United States on the 19th of January 1976, with a date of rank set at the 4th of July 1776.