A muggle is a person who is mathematically illiterate.
Specifically, it refers to those who do not realise that they are indeed embarrassingly mathematically unsophisticated, and insist on peppering their speech with mathematical-sounding phrases, like Q.E.D (on believing they have won an argument) or I had $n$ pints of beer last night (being under the impression that $n$ means "a big unspecified number").
It may be argued that it is unfair and elitist to call such people with such a demeaning cognomen, but we stand by our declaration on the grounds that it is frequently mathematicians who are equally unfairly derided by many in common society as being socially inept, awkward and lacking in sophistication. Such displays of utter balderdash in the media are exemplified in such productions as The Big Bang Theory, which, while attempting to make braininess accessible to the averagely intellectually endowed, end up propagating the very stereotype which it purports to debunk.
The word muggle is not exclusive to the field of mathematics; a notable usage of muggle is in hacker subculture to refer to a non-hacker.