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A Scottish person, often a thief when kitkats are involved and a liar, who is is generally hated by all members of any given group.
Emma: What do you guys think of fiona?
Pete: We all hate her she is an a real mcweehaggis

Matt: Is that a second kitkat?
Rian: chuck stones at her, she's a mcweehaggis!
by Expert17 July 29, 2011
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