1) v. to hang out with friends in a manner that is, in some way or another, more awesome than typical social engagement due to it's exceptional level of laxness.
2) adj. a) may describe a person who is exceptionally relaxed and care-free, or
b) a situation in which all participants are having a really good time, but typically with minimal stimulation (e.g. watching t.v. or talking about mundane things).
A: I've had a long day. I need to kick back.
B: That's cool. I'll invite some friends to my place and we can just hardcore chill.
A: That kid over there is hardcore chill!
B: The one with the Birkenstocks, dreads and a somewhat glazed expression?
A: ...No, the guy with the business suit and the rings under his eyes who looks like he hasn't seen fun since the last quarterly report came in (see:sarcasm).