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Noun: Any person(s) who occupy the bathroom for an extended period of time while you are defecating.

An Uncle Zack will usually enter the bathroom, be silent, and usually look in the mirror for extended periods. Be careful not to get caught peeking to see if the Uncle Zack has left.
I was done pinching my loaf but couldn't wipe because there was an Uncle Zack in the bathroom.
by ZackD83 December 02, 2009
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