Hardcore (emphasis on "o") is, in the original sense, a direction in rock music, considered the most difficult and downhole. The very word Hardcore can be translated from English as "hard style".
In a more general sense, "hardcore" is a complete tin, hell and horror. Especially popular among Internet residents is the saying: "Only [something], only hardcore!" In the meaning "More hell" or "We're not looking for easy ways."
Hardcore is listed: Slang