c. 1200, amiral, admirail, "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."
Amiral de la mer "commander of a fleet of ships" is in late 13c. Anglo-French documents. 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 Arabic word was later Englished as emir.
As amīr is constantly followed by -al- in all such titles, amīr-al- was naturally assumed by Christian writers as a substantive word, and variously Latinized .... [OED]
Also in Old French and Middle English the word was further conformed to familiar patterns as amirauld, amiraunt. The unetymological -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 the name of a type of butterfly from 1720, according to OED possibly a corruption of admirable.
among Arabic or Muslim peoples, "chief of a family or tribe; a ruling prince," 1590s, from Arabic amir "commander" (see admiral).
"commanding naval officer," 1690s, probably via Dutch kommandeur from French commandeur, from Old French comandeor (see commander). The U.S. Navy rank was created 1862, above a captain, below a rear-admiral.
"hindmost part, the space behind or at the back," c. 1600, abstracted from rerewarde "rear guard, hindmost part of an army or fleet" (mid-14c.), from Anglo-French rerewarde, Old French rieregarde, from the Old French adverb riere "behind" (from Latin retro "back, behind;" see retro-) + Old French garde "guardian" (see guard (n.)).
Earliest use often is specifically military, "hindmost body of an army or fleet." The English word in many early examples also may be a shortened form of arrear (see arrears), perhaps a misdivision of the arrears.
As a euphemism for "buttocks" it is attested by 1796. As an adverb, "behind," early 15c. As an adjective, "hindmost; pertaining to or situated in the rear," c. 1300, from Old French rere.
To bring up the rear "come last in order" is from 1640s. The naval rank of rear admiral is attested from 1580s, said to be so called from his originally ranking "behind" an admiral proper. Rear-view (mirror) is recorded from 1926. Rear-supper (c. 1300) was an old name for "last meal of the day."