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1.
"Diack" is a word deeply-rooted in alternate music culture. In their song "Dwarf Invasion", the lead singer of The Common Denominators says to an interuptting backup singer "Shut up, DIACK!"

"Diack" is a variation of dick. Instead of just meaning a penis, "diack" means a large penis.
"What a diack."
"Your mother sucked my diack!"
by BTF June 29, 2004
 
2.
Basically it means the same as dick but has more attitude so is quite good when used as an insult. How biatch is to bitch, diack is to dick.

^-^
omg your such a diack
by shawtyy^-^ April 15, 2009