A thudder is an enormous cock! Generally dressed to the side and causes problems when wearing anything shorter than knee length.
Its called the "Thudder" due to the noise it makes when it hits a hard surface (which due to its size is very often)
Ben and Mike standing at the Urinals (Ben is sneaking a look at Mikes cock
Ben: Hey Mike look at the size of my cock
Mike : zzzzzip, "THUD" *splash*
Ben: Oh thats why they call it the thudder
Mike then wakes up and realises the thudder dream needs to be let go!
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