A three-way contraction combining the words "it" "is" and "not".
mother: that's it, you're grounded for painting the dog purple!

kid: tisn't MY doing! My sister did it!
by GalaxyDancer April 17, 2005
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It isn't your ballsack and it isn't your bumhole. It's the area in between.

- see taint
Yeah, I have a 3-inch tisn't.
by FrioLoco November 13, 2004
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none exsistant tits
i need to by some padded bras. just look at my tisn'ts
by supermolo March 10, 2005
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'Tisn't - The smooth, hairless seam on the scrotum that divides the testicles from each other.

Southernism.
See: taint.
"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."
by SJ@24 July 29, 2009
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