admiral (n.) Look up admiral at Dictionary.com
c.1200, "Saracen commander or chieftain," from Old French amirail (12c.) "Saracen military commander; any military commander," ultimately from medieval Arabic amir "military commander," probably via Medieval Latin use of the word for "Muslim military leader." Meaning "highest-ranking naval officer" in English is from early 15c. The extension of the word's meaning from "commander on land" to "commander at sea" likely began in 12c. Sicily with Medieval Latin amiratus and then spread to the continent, but the word also continued to mean "Muslim military commander" in Europe in the Middle Ages.

The intrusive -d- probably is from influence of Latin ad-mirabilis (see admire). Italian form almiraglio, Spanish almirante are from confusion with Arabic words in al-. As a type of butterfly, from 1720, possibly a corruption of admirable.