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1.
A process in which two people defecate in the same toilet; achieved as one person straddles the other (like a spider swing), and defecates through the empty space in front of the other's crotch.
I couldn't hold it, so we took a spider shit.
by Gomerj84 January 16, 2010
 
2.
When two people cross legs (one person's left and one person's right) while sitting on the same toilet and shit at the same time. Completion without laughter means the two people are soulmates.
"Charles and I engaged in spider shits without laughing. I had no idea I was gay."
by Luis Espiezi March 08, 2012