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To 'mug someone off' when they believe to be 'mugging you off' first.
When someone tries to make you look foolish or put you in a embarrassing situation, that person is trying to 'mug you off'. The 'undermug' is where you are aware of this 'mugging' and counter act it with your own 'mugging' unbeknown to the original 'mugger'.
by GingyGinge December 02, 2009