Top Definition
a midly offensive term meaning a human being who quite simply has a penis inside their skull rather than a brain, therefore a willy brain. due to the lack of a brain they make foolish decisions and are very unfortunate, usually a person who is dullwitted and commonly unattractive to look at. it is taken from the latin "williusbrainionia" which roughly translates to willybrain.
Kate looked a total WILLYBRAIN in that sparkley bobblehat
by h.c. April 18, 2007
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