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1.
To fold or crease the brim of a baseball cap. It is usually done to ruin a flat brim hat and pisses people off.
hey mike, is this your new $35 phillies hat? How about a nice bunyak for it!
by Hockey11 July 21, 2010
 
2.
An old urkrainian insult meaning someone is being a silly bitch. The literal translation being outhouse or toilet or shitter.
Did you see Bob do that triple gainer and land on his fuckin head!?

Yah, what a crazy bunyak!
by Ukrainium June 29, 2012