That’s pronounced “pissah.” 
Point of order … Faulty logic.
When one says: "You ain’t shit."
What one is actually saying is: "By any and all comparisons, you have qualities which, when compared to fecal remains, fall below said standard of fecalocity. In essence, you aren’t even shit, metaphorically speaking, of course."
So you can see why they shortened it.
That is soooo much cooler than, “You ain’t shit.”
It’s like bollocks, which is the exact opposite of top bollocks, or the dog’s bollocks.