مُفْتٍ

Mufti comes from Arabic مُفْتٍ (muftī), an Islamic scholar qualified to issue religious rulings (fatwas). It's a serious title in Islamic jurisprudence.

But in British English, 'mufti' means civilian clothes worn by someone who usually wears a uniform. 'Mufti day' at British schools means no uniform required.

How did an Islamic legal title become slang for casual dress? Perhaps British officers in India, relaxing at home, joked that they were dressed 'like a mufti' — in flowing robes rather than uniform.

The connection is obscure and probably offensive. But the word stuck, and most British speakers have no idea of its origin.