This is the full read-only archive of the "Ask the Wordwizard" section of the original Wordwizard site. The responses to the questions originate from Jonathon Green, the compiler of the Cassell Dictionary of Slang and numerous other dictionaries.
If by 'mun' you mean the definition 'mouth', or in slang 'the chops' or the face, then it comes from Old Norse 'munnr' (modern Swedish is 'mun'; Danish is 'mund'). It can also be an abbrev. of 'money' and the name of a class of street-ruffians in the seventeenth century.ACCESS_POST_ACTIONS
Signature: Jonathon Green