We went in mufti—out of uniform, incog
The French say en pékin, and French soldiers call civilians pékins. An officer who had kept Talleyrand waiting, said he had been detained by some pékins. “What are they?” asked Talleyrand. “Oh,” said the officer, “we call everybody who is not military a pékin.” “And we,” said Tallyrand, “call everybody military that is not civil.” Mufti is an Eastern word, signifying a judge.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894