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1.
sin doos ism (n) 1. The simultaneous or near-simultaneous timing of defecation by two individuals without each other's knowledge, but sharing a bond such as friendship or homosexuality. 2. The creation of the urge to defecate by one individual, based on the knowledge that someone else has just defecated or has the urge to defecate.
Dude! My buddy and I were each dropping a deuce at the same time across town from each other ... we totally had syndeucism going on!

- Hey dude, I just laid the biggest turd.
- Oh my God, I must have syndeucism ... now I gotta take a shit too!
by Corkjob December 03, 2010