A Southwater Grunt is a Fresh water fish commonly found within the depths of the Great Lake Michigan.
The phrase 'Southwater Grunt!' on the other hand is rhyming slang (usually spoken with emphasis) to describe somebody as a CUNT. The phrase can be narrowed to 'Southwater' if used with the same emphasis.
The phrase has found to be extremely popular with the people of Essex, UK.
"oh and I see that Nick the other day, what a Southwater Grunt!"
Southwater Grunt can also be used to describe all Arsenal fans.
Mr Burns: Hey Arsenal played really well last night, I think I'm in love with Jack Wilshere...
Mr Binny: Shut up Mr Burns your such a southwater grunt