A derogatory term used in reference to hard rock music, referring to overuse of amplification on boring chord processions, extreme pounding of the bass drum with the pedal at a rapid rate, repetitive and often stupid lyrics and yelling (as opposed to singing) said lyrics.
Simply said, penis rock is music that is played by rock bands that have no talent and think that if they are loud enough, people will like them.
Metallica Mike: I'm listening to Metallica's new CD St. Anger.
Metallica Matt: Why are you listening to that penis rock when you could listen to Master of Puppets or And Justice for All.
Hardcore Hank: I'm listening to Slipknot's "Pulse of the Maggots."
Iron Maiden I Can't think of a name that starts with 'I': Why the hell do you like that penis rock? That is Slipknot's worst song.
Blood Brothers Berry: I like the Blood Brothers' early CDs, which involve much pounding and screaming.
The Spirit of Dimebag Darrell: That would be penis rock, but the lyrics are deep and meaningful (as only the God of Metal like me would understand). Pantera is awesome.