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1.
The act of one person taking a dump normally on a toilet while another person sits on their lap facing them and takes a dump through the space between the first persons legs.

A tandump can be performed either consensually or by ambush. In an ambush tandump, the assailant initiates a tandump by climbing over a stall wall and dropping down onto the unsuspecting pooper.
Ari: Hey Jacob, wanna take a tandump with me??
Jacob: Yeah of course! I call dibs on bottom!

************
Joe: Mike, why did you and Elaina break up?!?
Mike B: Her ex, Josh, told me that she really enjoyed tandumps, so I decided to surprise her with an ambush tandump! She threw up on me and threatened to call the police...
by AmericanSwagSociety December 08, 2010