Refers to the central, essential, or most important part of something, similar to English 'core'. Used in both concrete and abstract contexts.
Colloquial adjectival use describing something as extreme, intense, or deeply committed, often in the context of hobbies, fandoms, or subcultures.
This new technology will likely become the core of the company.
What is the core of this problem?
The Earth's core is extremely hot.
He is a hardcore gamer.
This band is supported by passionate fans.
He's a hardcore fan of that band.
Borrowed from English 'core'. The adjectival sense developed in Japanese colloquial usage, influenced by English expressions like 'hardcore'.