The word mufti leads a curious double life in English. In its original Arabic sense, a mufti (muftī) is an Islamic scholar qualified to issue fatwas — formal legal opinions on matters of Islamic law. In its British colloquial sense, mufti means civilian clothing worn by someone who normally wears a uniform. The connection between these two meanings is a piece of British military humor that has hardened into standard vocabulary.
The Arabic muftī derives from the root f-t-y, specifically the form aftā, meaning to give a formal legal opinion. A mufti is thus literally one who gives fatwas. In the Islamic legal tradition, the mufti occupied a position of considerable authority — a jurist whose expertise in Quranic interpretation, hadith, and legal precedent qualified them to provide authoritative guidance on questions of Islamic law.
The office of mufti was formalized under the Ottoman Empire, where the Grand Mufti (Şeyhülislam) served as the empire's highest religious authority. The Grand Mufti of Jerusalem, the Grand Mufti of Egypt, and similar offices continue to carry significant religious and political authority in the modern Islamic world.
The British military usage — 'in mufti' meaning in civilian clothes — first appeared in the early 19th century and is believed to originate in the theatrical tradition of portraying muftis in dressing gowns and tasseled caps. British officers changing out of their uniforms into comfortable civilian clothes supposedly compared themselves to actors playing muftis — lounging in flowing robes and soft headgear. The comparison was humorous and somewhat disrespectful, but the word stuck.
In British schools, 'mufti day' or 'non-uniform day' — when students are allowed to wear their own clothes instead of school uniform — preserves this military-derived usage. The phrase is standard in British, Australian, and South African English but largely unknown in American English.
The coexistence of these two meanings — solemn Islamic jurisprudence and casual British slang for civilian clothes — within a single word captures something of the colonial encounter between British and Islamic cultures. The military humor that created the second meaning could only have arisen in a context where British soldiers were familiar enough with Islamic institutions to make jokes about them.