hardcore: A branch of underground/indie music influenced by faster abrasive punk rock. Originating in the early 80’s as a variant of punk rock, hardcore evolved to a modern sound that can be compared to non commercial metal. The constant themes range from Straight Edge, to Politics, to Positive and Negative. ---- from Urban Dictionary
Hardcore can act as a modifier for a word or phrase, making it more severe; intense, relentless.
eg. This girl just spend a year train-hopping around the country surving only on dumpster-dived food and clothing. She was pretty damn hardcore. ---- from Urban Dictionary