Contrary to American definitions refering to female booty...in Australia "Donk" refers to a males cock. Makes for humorous listening to Soulja Boi's song "She Got A Donk".

Slang for a large penis -

A human mans erect penis

His penis is bigger than your mums, quite tall fucking dopey! ROCKS.....

A person with a penis equal to the size a donkey's penis

to get messed up so bad that you steal old peoples underwear when they go to take a dump.