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!