Like choad and taint, this word describes the wonderland between a person's genitalia and a-hole. "Shag," because it's hairy, and "nasty," because, well, look at it!
a) A Character on the Red Skelton Show.
b) A ladies' man.
c) {From the Greek} "Bolivus Shagnasticus", roughly meaning, "One who floods" (It was the title of a roman dictator roughly around 120 AD.)