One word, two meanings bridged by a joke — British officers compared their off-duty clothes to the flowing robes of Islamic scholars.
Plain clothes worn by someone who normally wears a uniform, or an Islamic scholar qualified to give rulings on religious law.
The Islamic scholar sense from Arabic muftī meaning one who gives a fatwa. The plain-clothes sense is British military slang, possibly humorous reference to the comfortable robes of a mufti Key roots: f-t-y (أفتى) (Arabic: "to give a formal legal opinion").