(verb) When one parties soft, they do not party hard, i.e they do not party at all, as it is impossible to party softly. If one were to party softly, one would be partying in an event akin to a tea party or seminar with an English lecturer.
Bouncer : "Oi, you look to young to be in here, you might party soft"
Man 2: "Mate, here's my fake i.d, don't read it, its pants, but I guarantee you, I'm here to party so hard"
Bouncer: "You're in."