When a military officer changes out of uniform into civilian clothes, he is in “mufti.”  But a mufti is a muslim scholar.  So, why?

Mufti  has been used by the British Army since 1816 and is thought to derive from the vaguely Eastern style dressing gowns and tasselled 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,”