The Generalissimo is the second officer in a Commandery of Knights Templar. His duty is to assist the Eminent Commander in the dispatch of business and, in the Commanders absence, preside over the Commandery. According to Encyclopedia of Freemasonry, the use of this title is fairly new and peculiar to American Masonic Templary.
Merriam-Webster defines Generalissimo as "the chief commander of an army" the first use of this word was in Italy around 1621. It originates from "generale", Italian for General, and the suffix -issimo, itself from Latin -issimus, meaning "utmost, to the highest grade".
On the symbolic interpretations of the jewel of the Generalissimo, Sir Knight George L. Marshall, Jr., wrote in the March 2011 edition of the Knights Templar magazine the following:
The jewel of the Generalissimo is shown at right. The metal is silver color for both the suspension bar and the jewel as it is for all remaining officers’ jewels as well. The jewel is a square, surmounted by a paschal Lamb. The square is to remind you that friendship and love should ever govern Freemasons and particularly Knights Templar. The square, as we know, is the jewel of office of the Worshipful Master of a Blue Lodge. Additionally, we are taught that the square is a symbol of morality. Since the square was used by operative masons to prove that angles were 90° or “right,” it naturally became an emblem of accuracy, integrity, and rightness. As stones are cut to fit into a building, so our acts and thoughts are built together into a structure of character badly or firmly and must be tested by a moral standard of which the simple square is a symbol.The paschal lamb atop the square has long been a symbol of Christ, apparently coming into use about the 15th Century. This symbol is sometimes referred to by the Latin name Agnus Dei (Lamb of God) or in heraldry, Holy Lamb. Heraldry defines it thus: a representation of a lamb passant [walking] having around its head a nimbus [halo] and supporting on the dexter [right] shoulder a crosslike staff bearing a flag argent [silver or white] charged with a cross gules [red].(Words in square brackets are supplied by me.) The term “paschal” originally related to the Passover, or Seder, of the Jews, and the paschal lamb was a lamb slaughtered and eaten on the eve of the first day of Passover. Jesus, the Lamb of God, was crucified during the time of Passover and so became our symbolic paschal lamb. The cross borne by the lamb represents the manner of his death and the flag His banner under which we as Templars have become His Knights. The walking or marching lamb illustrates that activity we should pursue in marching steadily onward to help spread His kingdom on earth.Thus, this jewel also reminds us to conduct ourselves with those high standards of morality and character which were taught by the Lamb of God while He walked among men and with that unconditional love that He exemplified in His sacrifice on Calvary.
Be thou Faithful unto Death,
and I will give Thee a Crown of Life