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The word originates from the Middle East and is Arabic: Mufti (مفتي) means an Islamic scholar who is an interpreter or expounder of Islamic law (Sharia), and is the active form of the Arabic afta, meaning "to judge". It has been used by the British army since 1816 and is thought to derive from the vaguely Eastern style dressing gowns and tasseled caps worn by off-duty officers in the early 19th century. Yule and Burnell's Hobson-Jobson: A Glossary of Colloquial Anglo-Indian Words and Phrases, and of Kindred Terms, Etymological, Historical, Geographical and Discursive (1886) notes that the word was "perhaps originally applied to the attire of dressing-gown, smoking-cap, and slippers, which was like the Oriental dress of the Mufti".
Mufti day (also known as Casual clothes day, Casual Friday, Color day, Own clothes day, Home clothes day, Plain clothes day, Non-uniform day, Mufting day, Free dress day, Civvies day, Dress down day, Uniform-free day) is a day where students and staff go to school in casual clothing instead of school uniform (or instead of smart clothes in the case of staff). In return, students are usually required to make a small donation which goes to a chosen charity or school fund-raising effort. This is found in many countries where students are required to wear uniform, including the United Kingdom, Ireland, Canada, Australia, New Zealand, South Africa, Zimbabwe, India, Pakistan and Bangladesh. It is particularly used in this way in state schools.