The d in admiral is a mistake, and the al is a misunderstanding. The word goes back to Arabic amir, "military commander" — the same word English later borrowed again, more accurately, as emir. In Arabic titles amir is constantly followed by al-, "the," as in amir-al-bahr, "commander of the sea." European writers heard that recurring amir-al- and took the whole thing to be one word, so the article got permanently glued on to a word it was never part of.
It came into English around 1200 through Old French as amiral, still meaning a Saracen commander on land; the naval sense seems to have started in twelfth-century Sicily and spread from there, reaching English by the early 1400s. The stray d appeared later, from the pull of Latin admirabilis — the source of admire — which the word simply resembled. So admiral is Arabic in origin, French in transmission, and Latin only in the one letter that has no business being there.
admiral (n.) 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 . also from c. 1200
admiral came in through Later and wider borrowings, and its spelling follows that family's conventions.
What gives it away on the page
| al- fused to the front of the noun | Arabic, usually via Spanish or Latin — al is the article, stuck on permanently.alcohol, algebra, algorithm, alkali, almanac, admiral |
Later and wider borrowings in the spelling guide · guess another word's family