Hard refuse matter, such as fragments of demolished walls, that may be re-used in the foundations of new structures (paraphrasing the _Oxford English dictionary_ http://dictionary.oed.com/cgi/entry/50050166?search_id=y2PI-exIYWn-16920&query_type=word&queryword=hardcore&result_place=1&first=1&sort_type=alpha&hilite=50050166se1&max_to_show=10)

Sexually explicit material that graphically depicts the genitals and sexual acts.

stubbornly resistant to change or improvement; "hard-core addicts"

intensely loyal; "his hard-core supporters"

Hardcore porn shows actual penetration during sexual acts involving 2 or more porn stars.

Refers to sexual acts involving two or more porn stars.

Hardcore is a 1979 film written and directed by Paul Schrader, and starring George C. Scott.