corruption of Amir-al. Milton, speaking of Satan, says:
His spear (to equal which the tallest pine Hewn on Norwegian hills, to be the mast Of some tall amiral, were but a wand) He walked with.
The word was introduced by the Turks or Genoese in the twelfth century, and is the Arabic Amir with the article al (lord or commander); as Amir-al-ma (commander of the water), Amir-al-Omra (commander of the forces), Amir-al-Muminim (commander of the faithful).
English admirals used to be of three classes, according to the colour of their flag—
The distinction was abolished in 1864; now all admirals carry the white flag.
Admirals are called Flag Officers.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Admiral from Infoplease: