Partying serious is much different
than partying hard. Partying serious is the much more sophisticated
version of partying; it is to keep classy company of the same gender (ladies and gentlemen), whilst in robes, basking in presence of leather bound books, a fireplace, cigars and/or pipes, and of course fine, fine cognac and/or wine.
"We bring the girls in later." Brandon Boyd
The interview where "party serious" can be clearly explained is located here:
start at 2:06, ENJOYincubus