"Daddy, what's this line on my scrotum?"
"Why, that's where Da Lawd done sewed up yo' nutsack after he inserted yo' balls.
'Tisn't yo' left nut and 'tisn't yo' right."
The small stretch of skin on a males genitalia between the ball sack and the asshole. It isn't (tisnt) part of the sack and it isn't (tisnt) part of the asshole.