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A Knacker Flap is the large flap of skin prevailing from the gut that hangs over a males genitals, making them completely invisible when naked. The sighting of a Knacker Flap usually occurs in older obese men, and is usually tucked into their shirt. It is the male version of the "gunt" which is that of a female Knacker Flap.
Holy shit, that dude's got a mean Knacker Flap!

Omg is that guy naked? Where's his privates? Oh they're underneath his Knacker Flap.
by purplejane September 08, 2014
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