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c.1200, "Saracen commander," from Old French amirail (12c.) "Saracen military commander; any military commander," probably ultimately from Arabic title amir-ar-rahl "chief of the transport," officer in the Mediterranean fleet, from amir "leader;" influenced by Latin ad-mirabilis (see admire).
Italian form almiraglio, Spanish almirante are from confusion with Arabic words in al-. Meaning "highest-ranking naval officer" is from early 15c. As a type of butterfly, from 1720, possibly a corruption of admirable.