Green’s Dictionary of Slang

upper decker n.

[as coined/defined in the film A Dirty Shame (2004), by John Waters]

one who defecates in the upper tank of a toilet, the stool then reveals itself in the toilet bowl when the next user flushes.

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[UK]Guardian G2 27 Sept. 16/3: His band of ‘frottagists’, ‘Roman-shower enthusiasts’, ‘Funchists’, ‘tromboners’ and ‘upper-deckers.’.