I believe the term "hardcore" was originally coined in the late 1960's or early 1970's to describe the poorest of the poor, those whom welfare programs fail.
I myself came to know the word in the mid 1980's to describe a style of punk rock typified by loud guitars and incomprehensible lyrics. Hardcore Punk Rock was around since the late 1970's at the latest, when differing forms of punk started to emerge. The word as such was probably coined in distinction with pop-punk. Since pop-punk was (is) usually propagated by the bourgeoise, the word 'hardcore' here could as easily describe the social class of those in the hardcore scene, as its participants' unflagging dedication to the punk rock lifestyle. The phenomenon of selling out will never reach those in the hardcore scene.