Heropanti is a made up kind of word, though people use it randomly. As such the word is made up of hero and suffix -panti, which is for -ness, or rather, a trait that someone shows. For example, someone who shows madness, is doing paagalpanti, paagal being mad.
So heropanti is hero-ness, basically showing off oneself as a filmy hero, and probably do all the action and drama that movie heroes do onscreen.
Heropanti in itself is a derogatory kind of word.