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Saying "Mul" whilst slapping your cheek indicates you think the person you are doing it to is an idiot.

You can also stick your tounge in your bottom lip so as to closely resemble a retarded person.

Mul comes from the extended phrase Katyulamulickiawi - which has its roots in Maori language of New Zealand.
David: "O is for Awesome."

Phil: "MUL!!!!"
by Eddie Munsta January 07, 2008
when you drink or smoke to much and get that hangover or just fucked up feelin
ayo i killed dat whole bottle i'm mul
by ra August 13, 2003
same as moulinyan; pejorative Italian slang for a black person
Hey Guido, he's a mul!
by elihu root July 29, 2006